BIOLOGY TOPICAL: Digestive and Excretory Systems Test Time: 21 Minutes* Number of Questions: 16 * The timing restrictions for the science topical tests are optional If you are using this test for the sole purpose of content reinforcement, you may want to disregard the time limit MCAT DIRECTIONS: Most of the questions in the following test are organized into groups, with a descriptive passage preceding each group of questions Study the passage, then select the single best answer to each question in the group Some of the questions are not based on a descriptive passage; you must also select the best answer to these questions If you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain Indicate your selection by blackening the corresponding circle on your answer sheet A periodic table is provided below for your use with the questions PERIODIC TABLE OF THE ELEMENTS H 1.0 He 4.0 Li 6.9 Be 9.0 B 10.8 C 12.0 N 14.0 O 16.0 F 19.0 10 Ne 20.2 11 Na 23.0 12 Mg 24.3 13 Al 27.0 14 Si 28.1 15 P 31.0 16 S 32.1 17 Cl 35.5 18 Ar 39.9 19 K 39.1 20 Ca 40.1 21 Sc 45.0 22 Ti 47.9 23 V 50.9 24 Cr 52.0 25 Mn 54.9 26 Fe 55.8 27 Co 58.9 28 Ni 58.7 29 Cu 63.5 30 Zn 65.4 31 Ga 69.7 32 Ge 72.6 33 As 74.9 34 Se 79.0 35 Br 79.9 36 Kr 83.8 37 Rb 85.5 38 Sr 87.6 39 Y 88.9 40 Zr 91.2 41 Nb 92.9 42 Mo 95.9 43 Tc (98) 44 Ru 101.1 45 Rh 102.9 46 Pd 106.4 47 Ag 107.9 48 Cd 112.4 49 In 114.8 50 Sn 118.7 51 Sb 121.8 52 Te 127.6 53 I 126.9 54 Xe 131.3 55 Cs 132.9 56 Ba 137.3 57 La * 138.9 72 Hf 178.5 73 Ta 180.9 74 W 183.9 75 Re 186.2 76 Os 190.2 77 Ir 192.2 78 Pt 195.1 79 Au 197.0 80 Hg 200.6 81 Tl 204.4 82 Pb 207.2 83 Bi 209.0 84 Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra 226.0 89 Ac † 227.0 104 Unq (261) 105 Unp (262) 106 Unh (263) 107 Uns (262) 108 Uno (265) 109 Une (267) * 58 Ce 140.1 59 Pr 140.9 60 Nd 144.2 61 Pm (145) 62 Sm 150.4 63 Eu 152.0 64 Gd 157.3 65 Tb 158.9 66 Dy 162.5 67 Ho 164.9 68 Er 167.3 69 Tm 168.9 70 Yb 173.0 71 Lu 175.0 † 90 Th 232.0 91 Pa (231) 92 U 238.0 93 Np (237) 94 Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259) 103 Lr (260) GO ON TO THE NEXT PAGE as developed by Digestive and Excretory Systems Test Passage I (Questions 1–5) The kidneys regulate hydrogen ion (H + ) concentration in extracellular fluid primarily by controlling the concentration of bicarbonate ion (HCO3–) The process begins inside the epithelial cells of the proximal tubule, where the enzyme carbonic anhydrase catalyzes the formation of carbonic acid (H2CO 3) from CO and H2O The H2CO then dissociates into HCO3– and H+ The HCO3– enters the extracellular fluid, while the H+ is secreted into the tubule lumen via a Na+ –H + countertransport mechanism, which takes advantage of the Na+ gradient established by the Na+–K+ pump Since the renal tubule is not very permeable to the H C O – filtered into the glomerular filtrate, the reabsorption of HCO3– from the lumen into the tubular cells occurs in a roundabout way Carbonic anhydrase causes this HCO3 – to combine with the secreted H+ , forming H2CO The H2CO then dissociates into CO2 and H2O The H2O remains in the lumen, while the CO2 enters the tubular cells Looking at Figure 1, it can be seen that inside the cells, every H+ secreted into the lumen is countered by an HCO 3– entering the extracellular fluid Thus, the basic mechanism by which the kidneys regulate body fluid pH is by the titration of H+ with HCO3– tubular cell K+ HCO3 Na+ Na+ HCO3 + H+ H+ + HCO3 [ H2CO3 CO2 Na+ + HCO3 2K+ 3Na+ H2O + CO2 [ ] carbonic anhydrase A B C D Facilitated diffusion Passive diffusion Active transport Endocytosis Administration of acetozolamide to a patient would result in an increase in the excretion of: I Na+ II HCO3– III H + IV H2O A II only B IV only C I, II, and IV only D I, II, III, and IV The kidneys excrete acidic urine when: tubule lumen extracellular fluid Based on information in the passage, which of the following is the mechanism by which Na + is transported into the tubular cells? ] carbonic anhydrase H2CO3 CO2 + H2O A more hydrogen ions are secreted into the tubular lumen than bicarbonate ions are filtered into the glomerular filtrate B more bicarbonate ions are filtered into the glomerular filtrate than hydrogen ions are secreted into the tubular lumen C more hydrogen ions are filtered into the glomerular filtrate than bicarbonate ions are secreted into the tubular lumen D more bicarbonate ions are secreted into the tubular lumen than hydrogen ions are filtered into the glomerular filtrate Figure The drug acetozolamide is a potent noncompetitive inhibitor of carbonic anhydrase Acetozolamide is a diuretic—a drug that increases the rate of urine formation and thereby increases the excretion of water and other solutes Diuretics can be used to maintain adequate urine output or to rid the body of excess fluid KAPLAN GO ON TO THE NEXT PAGE MCAT Which of the following hormones would have the opposite effect on blood volume as acetozolamide? A B C D ADH FSH Calcitonin Somatostatin Spironolactone is an adrenocorticosteroid that acts as a diuretic by inhibiting aldosterone A patient taking this drug would most likely have a(n): A increased blood volume and a decreased plasma concentration of Na+ B increased blood volume and an increased plasma concentration of Na+ C decreased blood volume and a decreased plasma concentration of Na+ D decreased blood volume and an increased plasma concentration of Na+ GO ON TO THE NEXT PAGE as developed by Digestive and Excretory Systems Test Passage II (Questions 6–10) Despite its bad reputation, cholesterol plays a vital role in health maintenance Cholesterol serves as a precursor for bile salts, steroid hormones, and other specialized molecules, and is a constituent of membranes Cholesterol is primarily supplied by the diet or synthesized by the liver, though all cells have cholesterol biosynthesis capability Most of the cholesterol in plasma is carried by LDL (low-density lipoproteins), which is the product of VLDL (very low-density lipoproteins) metabolism Almost every body cell has LDL membrane receptors Binding of LDL to these receptors triggers an invagination of the cell membrane and engulfment of the LDL-cholesterol complex Inside the cell, enzymes break down the LDL, releasing the cholesterol for incorporation into membranes or other specialized molecules Intracellular free cholesterol inhibits its own biosynthesis by inhibiting the rate-limiting enzyme involved in the process Thus, dietary and liver-produced cholesterol is utilized before the cell produces its own When intracellular free cholesterol is low, the cells produce more LDL receptors in order to increase cholesterol uptake from the plasma Another carrier, called HDL (high-density lipoproteins), lowers plasma cholesterol concentration by binding to free cholesterol from dying plasma membranes These HDL-cholesterol complexes are metabolized by liver cells, yielding free cholesterol that is secreted into bile The bile eventually empties into the duodenum, where it is reabsorbed from the intestine or excreted into the feces Alternatively, liver cells may metabolize the free cholesterol into bile acids The bile acids then pass through the bile duct and into the small intestine, where the biliary cholesterol is reabsorbed from the intestine or excreted into the feces Which cell organelle is responsible for the enzymatic degradation of LDL-cholesterol complexes? A B C D Normal VLDL production by the liver is inhibited by the ingestion of high concentrations of the vitamin niacin Which of the following would most likely occur in an individual consuming excess niacin? A Decreased bile cholesterol levels B Decreased cholesterol secretion into the small intestine C Increased activity of cellular cholesterolsynthesizing machinery D Increased plasma concentration of VLDL Research has shown that ingestion of saturated fatty acids (SFAs) tends to raise plasma cholesterol both by inhibiting the conversion of free cholesterol to bile acids and by stimulating cholesterol biosynthesis Which of the following would best account for these findings? A Chylomicrons containing SFAs inhibit the ratelimiting enzyme involved in cellular cholesterol biosynthesis B SFAs resemble cholesterol molecules to the extent that the body is often unable to differentiate between them C Chylomicrons containing SFAs bind to LDL receptors, thereby blocking the receptor-mediated endocytosis of LDL-cholesterol complexes D SFAs mimic the action of HDL molecules, thereby increasing cholesterol transport and increasing cholesterol metabolism in liver cells Lysosomes Mitochondria Endoplasmic reticulum Ribosomes GO ON TO THE NEXT PAGE KAPLAN MCAT An individual with an average daily intake of cholesterol discovers that she has abnormally high plasma cholesterol Which of the following steps would be effective in lowering her plasma cholesterol concentration? I II III IV A B C D Increase bile cholesterol Increase plasma HDL level Decrease plasma HDL level Decrease dietary cholesterol intake IV only II and IV only III and IV only I, III, and IV only Familial hypercholesterolemia is a disease in which certain afflicted individuals maintain a plasma cholesterol level five times higher than the normal level Which of the following best explains this phenomenon? A Afflicted individuals have abnormally low cholesterol requirements for normal body functioning B Afflicted individuals are unable to produce the rate-limiting enzyme involved in cellular cholesterol biosynthesis C Afflicted individuals have defective livers that metabolize LDL-cholesterol complexes as well as HDL-cholesterol complexes D Afflicted individuals lack the gene that codes for LDL receptors GO ON TO THE NEXT PAGE as developed by Digestive and Excretory Systems Test Questions 11 through 16 are NOT based on a descriptive passage 11 If the hormone secretin were not released when chyme entered the small intestine, which of the following would NOT occur? A B C D Secretion of enterogastrone Secretion of cholecystokinin Secretion of insulin Secretion of bicarbonate ion Children who develop acute glomerulonephritis, in which the renal glomeruli are inflamed and therefore fail to filter adequate quantities of fluid, would be expected to develop all of the following EXCEPT: A B C D an excess of extracellular fluid in body tissue a high concentration of urea in the blood a decreased urine output a high concentration of sodium in the urine Vitamin K cannot be synthesized by human cells While the amount of vitamin K in ingested foods is not normally sufficient to maintain adequate blood coagulation, the normal plasma concentration of vitamin K is sufficient Which of the following best accounts for this apparent contradiction? The “hunger pangs” that may be experienced in the stomach 12 to 24 hours after the last ingestion of food are due to: A the emptying of the stomach contents into the small intestine through the pyloric sphincter B peristaltic contractions of the stomach C the excess secretion of mucus in the stomach D the highly acidic environment of the stomach A Vitamin K is synthesized in the liver using hormones absorbed from ingested plant cells B Vitamin K is a byproduct of protein degradation during digestion C Degraded vitamin K is reabsorbed into the blood from the kidneys D Vitamin K is synthesized by bacteria that inhabit the colon Which of the following would most likely cause malabsorption in the small intestine? In the process of fat emulsification, bile salts make fats more susceptible to the action of lipases by: A transporting fat globules to the region of lipase activity B increasing the surface area of the fat globules C functioning as a catalyst to the lipases D lowering the pH of the small intestine A Presence of extremely dilute chyme in the small intestine B Destruction of the intestinal villi C Deceased secretion of HCl in the stomach D Presence of alkaline fluid in the small intestine END OF TEST KAPLAN MCAT ANSWER KEY: A C A A C 10 A C C B D 11 12 13 14 15 D B B D D 16 B as developed by Digestive and Excretory Systems Test DIGESTIVE AND EXCRETORY SYSTEMS TEST TRANSCRIPT Passage I (Questions 1-6) The correct answer is choice A Referring to both Figure and the passage, you see that H+ is secreted into the lumen via a Na+-H+ countertransport mechanism After carbonic anhydrase catalyzes the formation of carbonic acid inside the tubular cell, the carbonic acid spontaneously degrades into bicarbonate ion and H+, causing a buildup of H+ within the cell The H+ is secreted into the lumen in exchange for Na+ The Na+ binds to a carrier protein on the luminal side of the cell membrane, while at the same time, an H+ binds to the opposite side of the same carrier protein Since there is a greater concentration of Na+ outside the cell than inside, due to the Na+ -K+ pump, Na+ moves down its concentration gradient into the cell This movement supplies the energy for moving the H+ into the tubular lumen Therefore, it is said that the Na+ moves into the cell via facilitated diffusion; no energy is required because movement is with the concentration gradient, not against it Thus, choice A is the correct answer Choice C is wrong because although active transport requires a protein carrier, it also requires APT Choice D can also be eliminated Endocytosis is the uptake of extracellular material via invagination of the plasma membrane Clearly, that's not occurring here In this case, one ion is being exchanged for another ion Choice B, passive diffusion, is also incorrect Passive diffusion is the movement of an ion down its concentration gradient without the aid of a carrier protein and the countertransport of another ion Again, the correct answer is choice A The correct answer is choice C The key to solving this question is understanding the effects of carbonic anhydrase From Figure 1, we know that carbonic anhydrase is found in both the lumen and within the tubular cells of the kidney In the lumen, hydrogen ions and bicarbonate ions are converted to carbonic acid, which then dissociates into carbon dioxide and water This reaction is catalyzed by carbonic anhydrase The water remains in the lumen, while the carbon dioxide enters the cells Inside the cells, carbonic anhydrase catalyzes the formation of carbonic acid from water and carbon dioxide The carbonic acid dissociates into a bicarbonate ion and a hydrogen ion The bicarbonate ion enters the extracellular fluid, while the hydrogen ion is secreted into the lumen in exchange for a sodium ion According to the passage, acetozolamide is an inhibitor of carbonic anhydrase Thus, administering acetozolamide would prevent those reactions that are catalyzed by carbonic anhydrase And this would mean that the sodium ion-hydrogen ion countertransport would come to a halt, as would the reabsorption of bicarbonate ion from the lumen into the tubular cells Since the fluid remaining in the lumen is eventually excreted in the urine, acetozolamide would result in a decrease in urinary hydrogen ion, an increase in urinary bicarbonate ion, and an increase in urinary sodium ion This increase in urine osmolarity would draw water into the lumen, to counter the increase in lumen fluid osmolarity Thus, there will be an increase in the urinary amounts of Roman numerals I, II, and IV, and a decrease in Roman numeral III, and so choice C is the right answer Once you had determined that Roman numeral I was a component of the correct answer, you should have ruled out choices A and B, narrowing it down to C and D Again, choice C is correct Choice A is correct According to the passage, the secretion of hydrogen ions into the lumen is countered by the movement of bicarbonate ions into the extracellular fluid from the filtrate, and that this titration is the means by which the kidneys regulate body fluid pH Acidity is a function of hydrogen ion concentration Therefore, when the kidneys excrete acidic urine, it means that there is a high concentration of hydrogen ions in the urine relative to the concentration of bicarbonate ions Since urine is formed from the fluid in the tubule lumen, acidic urine results when the lumen fluid has a high hydrogen ion concentration relative to the bicarbonate ion concentration This means that more hydrogen ions are secreted into the tubule than bicarbonate ions are filtered into the lumen from the glomerulus Thus, choice A is correct and choice B is wrong Now even if you weren't able to figure out the origin of acidic urine, you should have been able to narrow it down to choice A and B right off the bat based on Figure alone As you can clearly see, it is the hydrogen ions that are secreted INTO the tubular lumen, thereby eliminating choice C and D Again, choice A is the correct answer The correct answer is A Answering this question requires an understanding of the effect acetozolamide and various hormones have on the body's fluid status Acetozolamide inhibits carbonic anhydrase and thus inhibits both the excretion of hydrogen ions into the lumen and the re-absorption of bicarbonate The excretion of hydrogen ions into the lumen is important because it provides the means by which sodium is taken up into the tubular cell If no hydrogen ions are produced in the tubular cell due to the inhibition of carbonic anhydrase, the sodium cannot be taken up and so remains within the lumen, increasing its osmolarity Water will diffuse back into the lumen due to the increased osmolarity of the lumen, and thus, urine volume will increase Indeed, this is the way in which this diuretic acts to rid the body of excess fluid The hormone with the opposite effect then, is one that causes a decrease in water excretion In other words, this hormone must cause an increase in water re-absorption and ADH is the hormone we're looking for ADH, or antidiuretic hormone, increases the re-absorption of water in the kidneys Even its name, ANTI-diuretic hormone, should have made this question a dead giveaway, since acetozolamide is a diuretic Thus choice A is the correct answer Choice B is incorrect because FSH, or follicle-stimulating hormone, causes the maturation of an ovum in preparation for ovulation Choice C, calcitonin, is also incorrect, since calcitonin is a hormone that decreases plasma calcium Calcitonin is secreted by the thyroid gland in response to high plasma calcium ion concentration Similarly, choice D, somatostatin, is also incorrect Somatostatin is secreted by KAPLAN MCAT the hypothalamus and is a potent inhibitor of many other hormones, including: growth hormone, TRH (thyroid-releasing hormone), ACTH, insulin, glucagon, gastrin, and renin Once again, the correct answer is choice A Choice C is the correct answer Coming up with the answer to this question required no information from the passage itself According to the question stem, spironolactone is a diuretic that inhibits aldosterone Aldosterone is the hormone that causes an increase in the re-absorption of sodium ions from the lumen of the nephron and an increase in the secretion of potassium ions into the lumen of the nephron Thus, aldosterone increases the plasma concentration of sodium ions Therefore, a drug that inhibits aldosterone will inhibit the re-absorption of sodium ions and will thereby decrease the plasma concentration of sodium ions Based on that piece of information, you know that we can eliminate choices B and D To distinguish between choices A and C, you need to know about aldosterone's effect on blood volume Osmolarity is the number of solute particles in a certain volume of liquid And you know that if you have two compartments divided by a membrane permeable to water, water will diffuse from the area of lower osmolarity to the area of higher osmolarity Sodium is re-absorbed from the tubular filtrate into the tubular cells, and eventually enters the bloodstream This movement of sodium from the tubular lumen to the cells, and finally to the blood results in an increase in the osmolarity of these regions, resulting in a concomitant movement of water So where the sodium ions go, the water follows, which means than an increase in the plasma concentration of sodium will be accompanied by an increase in blood volume Therefore, when you inhibit aldosterone, not only you inhibit the re-absorption of sodium, but you inhibit the movement of the water as well, resulting in a decrease in blood volume And so, choice C is the correct answer Even if you didn't remember the effects of aldosterone there was still a way to get the right answer You're told in the question stem that spironolactone is a diuretic And from the passage you know that diuretics increase the loss of water and other solutes Therefore, upon administration of a diuretic, you would expect a decreased plasma concentration of solutes along with a decrease in blood volume, since urine excretion has increased So by this line of reasoning, it is obvious that choice C must be the correct answer Passage II (Questions 6-10) The correct answer is choice A To answer this question, you needed to remember the basic functions of these four cell organelles Lysosomes are the garbage disposal system of cells Lysosomes make use of hydrolytic enzymes to accelerate the breakdown of unwanted proteins, polysaccharides, nucleic acids, and lipids In this passage, we have lysosomes separating the LDL from the cholesterol molecules, thereby freeing the cholesterol for use by the cell Thus, choice A is right Choice B is wrong because the primary function of mitochondria is cellular respiration the process by which usable energy is derived from food materials and stored in the high-energy phosphate bonds of ATP Choice C is wrong because the endoplasmic reticulum (ER) serves as the main intracellular transport mechanism for molecules, especially proteins While some cells contain large amounts of ER, some of which has associated degradative enzymes, intracellular degradation is not the primary function of most cellular ER and certainly cannot be considered the main degradative source for the breakdown of LDLcholesterol complexes Ribosomes, choice D, either float freely within the cytoplasm or are connected to the membranes of the ER (rough endoplasmic reticulum) Ribosomes are responsible for translating messenger RNA transcripts into peptide chains Thus, choice D is wrong Again, choice A is correct Choice C is the correct answer VLDL is synthesized by the liver, but it is soon cleaved into LDL by removal of a lipid moiety Thus, if the vitamin niacin interferes with VLDL production, which is exactly what happens according to the question stem, then ingestion of excessive amounts of niacin will effectively reduce the LDL concentration in that individual's plasma Thus, right off the bat, you should have eliminated choice D, because VLDL plasma concentration will obviously be decreased, not increased An individual with decreased amounts of available LDL will have fewer LDL-cholesterol complexes The net result is that cholesterol uptake will decrease, and the rate-limiting enzyme that controls cellular cholesterol biosynthesis will not be inhibited This will result in increased activity of the cellular cholesterol-synthesizing machinery to compensate for the apparent lack of dietary cholesterol Thus, choice C is correct Choice A is incorrect because, as stated in the passage, bile cholesterol levels are primarily a function of HDL concentration Since VLDL is not the precursor of HDL, a decrease in VLDL production would not affect HDL levels On the contrary, if less cholesterol is made available to cells, and more cholesterol is synthesized intracellularly, then the resulting increase in plasma cholesterol levels would initiate plasma cholesterol clearance mechanisms For this reason, liver cells would secrete more cholesterol into the bile, where it could be excreted into the feces So, choice A is wrong Choice B is also wrong, for many of these same reasons HDL is responsible for controlling how much cholesterol is transported into liver cells, where it is broken down into bile acids and secreted into the small intestine VLDL concentration does not regulate HDL concentration Also, since a low LDL level would result in increased plasma cholesterol, this would result in an increase in cholesterol secretion into the small intestine Again, choice C is the correct answer Choice C is correct To get this question right, you need to understand the underlying factors responsible for the regulation of plasma cholesterol levels that are described for you in the passage The question presents a situation in which the conversion of cholesterol into bile acids is inhibited and cholesterol synthesis is stimulated Choice C is correct because it best accounts for these two effects You might have remembered that chylomicrons are small lipid droplets that are very hydrophobic, and thus are able to absorb similarly hydrophobic molecules such as SFAs, effectively removing them from 10 as developed by Digestive and Excretory Systems Test circulation until they are needed If the chylomicrons were to bind to LDL receptors on cell membranes, they would compete with cholesterol molecules, which would prevent or inhibit cholesterol from being absorbed into cells With less cholesterol present inside the cells, the rate-limiting enzymes of these cells would not be inhibited, and so intracellular cholesterol biosynthesis would increase So, choice C is the right answer Choice A is incorrect, because if chylomicrons containing SFAs actually inhibited the rate-limiting enzyme involved in cellular cholesterol biosynthesis, then this enzyme would not be effective in carrying out its intended function and cholesterol biosynthesis would be impaired Since the research showed cholesterol biosynthesis to be stimulated rather than impaired, choice A is wrong Choice B is also incorrect because if the SFAs resembled cholesterol molecules to the extent that the body could not distinguish between the two, then ingested SFAs would affect the body in a manner similar to ingested cholesterol They would be absorbed via the LDL receptor-mediated endocytosis and would then inhibit the rate-limiting enzyme within each cell, thus causing intracellular cholesterol biosynthesis to be impaired Since this would contradict the research data, choice B must be wrong Finally, if SFAs mimicked HDL action to the extent that they could bind with free-floating cholesterol, this would facilitate the transport of cholesterol into the liver cells This, in turn, would cause the liver cells to metabolize increased amounts of cholesterol Bear in mind, however, that these liver cells convert cholesterol into bile acids, the very process that is inhibited by SFAs according to the question stem If choice D were true, cholesterol conversion to bile acids would be stimulated, not inhibited Therefore, choice D is wrong Again, choice C is correct Choice B is correct You're looking for the options that result in a decrease in plasma cholesterol According to the passage, your body has two main ways to obtain cholesterol: it's either ingested or synthesized intracellularly Obviously, by decreasing your ingestion of dietary cholesterol, you would restrict your body's access to cholesterol, thus decreasing plasma cholesterol Thus, Option IV should be included in your answer Unfortunately, this doesn't narrow the field at all, since Option IV appears in all choices Okay, what about the others? HDL complexes with free cholesterol, which eventually results in the excretion of some of this cholesterol A person with abnormally high plasma cholesterol would therefore be particularly responsive to therapy in the form of an increase in plasma HDL This would allow more cholesterol to be bound, metabolized by the liver, and excreted through the feces Thus, Option II would also be effective in lowering her plasma cholesterol level So this means that choice B must be the right answer since it's the only one with both of these options Option I would not be effective in lowering plasma cholesterol because increasing bile cholesterol would necessarily involve increasing plasma cholesterol, since bile cholesterol is derived from plasma cholesterol that found its way into liver cells Option III would also be ineffective, because lowering HDL levels means that cholesterol uptake by liver cells would decrease Again, choice B is correct 10 Choice D is correct From the passage, you know that LDL reduces plasma cholesterol levels by binding to and transporting cholesterol into cells, where it is metabolized and used in the synthesis of a variety of molecules Remember, though, that for the LDL-cholesterol complex to traverse the cell membrane, it must first bind to an LDL receptor As choice D suggests, if a person did not possess the gene that codes for the LDL receptor, then they would not be able to synthesize any LDL receptors This would result in an elevated plasma cholesterol for two reasons First, LDLs would not be able to reduce plasma cholesterol because they would not be able to transport cholesterol into cells Second, if cholesterol doesn't get into cells, then the rate-limiting enzyme that controls intracellular cholesterol biosynthesis would not be inhibited Thus, even though plasma cholesterol might be high, the body would continue synthesizing cholesterol unchecked, resulting in hypercholesterolemia For this reason, choice D is correct Choice A is wrong because there's simply no information in the passage to lead you to such a conclusion You don't even know if there is such a thing as a decreased need for cholesterol After all, we all need cholesterol for our membranes and other biological structures Choice A, therefore, is definitely not the best explanation for the symptoms of familial hypercholesterolemia, and is thus wrong Choice B is wrong, because if the rate-limiting enzyme for cellular cholesterol biosynthesis was not produced, then that individual would simply be unable to produce cholesterol naturally Since this would not result in abnormal elevations of plasma cholesterol, choice B is a poor explanation for hypercholesterolemia Choice C is wrong, because having a defective liver that metabolized cholesterol from both LDL- and HDL-cholesterol complexes wouldn't stop other body cells from uptaking the LDL complexes, which would result in the inhibition of intracellular cholesterol biosynthesis The net result of this defective liver would be lowered plasma cholesterol levels, thus choice C is incorrect Again, choice D is the right answer Discretes (Questions 11-16) 11 The correct answer is choice D Secretin is the hormone secreted by the duodenum when the acidic chyme enters the small intestine Secretin stimulates the pancreas to secrete pancreatic juice, which contains a high concentration of bicarbonate ion and is therefore alkaline enough to neutralize the chyme Neutralization of the chyme's acidity is particularly important for the activity of the small intestine's enzymes, which operate at a slightly basic pH and are denatured in an acidic environment Enterogastrone, choice A, is released by the duodenum when the chyme is fatty Enterogastrone slows the release of chyme into the small intestine by inhibiting stomach peristalsis, allowing more time to digest the fats Cholecystokinin (CCK), choice B, is secreted by the duodenum in response to the presence of chyme CCK stimulates the secretion of KAPLAN 11 MCAT pancreatic enzymes and the release of bile Insulin, choice C, is the hormone secreted by the pancreas in response to high glucose blood concentration; it is not directly involved in the digestive process Again, choice D is the right answer 12 The correct answer is choice B It is the peristaltic contractions of the stomach itself that causes hunger pangs Choice A has to be wrong because the emptying of the stomach contents into the small intestine through the pyloric sphincter occurs much earlier in the digestive process than the 12 to 24 hours following the last ingestion of food Choice C is wrong because in addition to lubricating food for ease of transport, mucus provides a major shell of protection for the stomach from its highly acidic environment and its digestive enzymes Excess mucus would not cause any sort of pain Choice D can be eliminated because the stomach's environment is normally highly acidic, and does not cause pain, and so there is no reason to think that the acid would cause pain 12 to 24 hours after the last ingestion of food (This does occur, however, in cases of peptic ulcer disease.) Again, choice B is correct 13 Choice B is the correct answer Emulsification, which occurs via the detergent action of bile salts, increases the surface area of fat globules exposed to the lipases Emulsification involves the breaking of fat globules down into smaller particles so that the lipases can act on their surfaces Since the lipases can only attach the fat globules on their surfaces, the action of bile salts greatly enhances the lipases' capacity for fat digestion Thus, choice B is correct Choice A is wrong because fat digestion occurs in the small intestine; the fat globules not have to be transported there Choice C is wrong because the only compound that comes close to acting as a catalyst for lipase is the protein colipase, without which lipase could not act on fats Choice D is wrong because bicarbonate ion is secreted into the small intestine for the sole purpose of raising pH, and countering the acidic chyme entering from the stomach Then environment of the small intestine needs to be slightly alkaline for proper digestion to occur Again, choice B is the right answer 14 The correct answer is choice D When the glomeruli are unable to filter adequate amounts of fluid, the major physiological effect is retention of salt and water, since these can no longer be excreted by the kidneys Urinary output decreases markedly (choice C) to less than 500 mL/day The quantity of salt and water in the extracellular fluid increases drastically and decreases in the urine, which leads to an excess of extracellular fluid in body tissue (choice A), a condition known as edema If the glomeruli are shut down, urea cannot be excreted either Thus, choice A and C are wrong and choice D is right And, another consequence of the glomeruli inflammation is the excessive accumulation of urea in the blood (choice B), a condition known as uremia So, choice B is also wrong Again, choice D is the right answer 15 The correct answer is choice D The vitamin K we need for blood coagulation is synthesized by the intestinal bacteria residing in our colons Vitamin K is required for the liver to synthesize blood factors VII, IX, and X, and prothrombin Deficiency of vitamin K causes hypoprothrombinemia, which manifests itself in defective coagulation of the blood Choices A, B, and C are clearly wrong, since they all involve human synthesis of production of vitamin K, and you're clearly told in the question stem that this is an impossible feat Again, choice D is the right answer 16 The correct answer is choice B The presence of the millions of small villi over the entire surface of the small intestine is crucial to its absorptive capacity The villi greatly enhance the surface area of the small intestine's absorptive membrane Extremely dilute chyme (choice A) presents little problem for the small intestine, because water is readily transported through the intestinal mucosa into the blood of the villi when there is sufficient osmotic pressure HCl is secreted in the stomach, not the small intestine If decreased secretion of HCl in the stomach caused the chyme entering the small intestine to be less acidic, this would not adversely affect intestinal absorption In fact, the digestive enzymes of the small intestine operate best at a slightly basic pH And so choice C is also wrong The presence of alkaline fluid in the small intestine is the norm, due to the secretion of bicarbonate ion into the small intestine Thus, choice D is wrong since this would obviously not result in malabsorption Again, choice B is the right answer 12 as developed by ... small intestine END OF TEST KAPLAN MCAT ANSWER KEY: A C A A C 10 A C C B D 11 12 13 14 15 D B B D D 16 B as developed by Digestive and Excretory Systems Test DIGESTIVE AND EXCRETORY SYSTEMS TEST. .. by Digestive and Excretory Systems Test Questions 11 through 16 are NOT based on a descriptive passage 11 If the hormone secretin were not released when chyme entered the small intestine, which... concentration of Na+ D decreased blood volume and an increased plasma concentration of Na+ GO ON TO THE NEXT PAGE as developed by Digestive and Excretory Systems Test Passage II (Questions 6–10) Despite