The esophagus is different from the GI tract because the esophagus has

Correct Answer: A

Rationale: The correct answer is A because the esophagus contains skeletal muscles at the superior end (upper part) for voluntary control during swallowing and smooth muscles at the inferior end (lower part) for involuntary peristalsis to move food to the stomach. This arrangement allows for both conscious and unconscious control of food movement. Now, let's analyze why the other choices are incorrect: B: The esophagus does have a tunica submucosa, which is important for support and nourishment of the mucosa. C: Villi and microvilli are found in the small intestine, not the esophagus, and they increase surface area for nutrient absorption. D: The esophagus does have a tunica mucosa, which contains mucous glands to aid in lubrication and protection of the lining.

Correct Answer: B

Rationale: The correct answer is B: cholecystokinin; low pH of the chyme. Cholecystokinin is released in response to the presence of fats and proteins in the duodenum. This hormone stimulates the gallbladder to release bile into the small intestine to aid in the digestion and absorption of fats. The low pH of the chyme triggers the release of cholecystokinin as it indicates the presence of undigested fats and proteins, signaling the need for more bile to aid in their digestion. Choices A, C, and D are incorrect because cholecystokinin is specifically released in response to the low pH of the chyme, not the high pH. Secretin, on the other hand, is released in response to the high pH of the chyme to stimulate bicarbonate release from the pancreas to neutralize the acidic chyme, not for bile secretion.

Correct Answer: D

Rationale: The correct answer is D: amylase. Amylase is the enzyme responsible for breaking down starch into maltose. It works by catalyzing the hydrolysis of the glycosidic bonds between glucose molecules in starch. Lipase (A) breaks down lipids, not carbohydrates. Maltase (B) is responsible for breaking down maltose into glucose. Lactase (C) breaks down lactose, not starch. Therefore, amylase is the correct enzyme for breaking down starch into maltose.

Correct Answer: C

Rationale: The correct answer is C because hemicelluloses, pectins, and inulin are indeed rapidly fermented to short-chain fatty acids in the colon, providing trophic effects. This is supported by scientific evidence. Choice A is incorrect because the main end products of fermentation of dietary fiber are short-chain fatty acids, not malic, succinic, and propionic acids. Choice B is incorrect as not all forms of fiber are completely fermented within 24 hours, and the time frame can vary. Choice D is incorrect because stool bulking is primarily attributed to soluble fibers, which hold water and increase stool bulk, rather than insoluble fibers like cellulose.

Correct Answer: D

Rationale: The correct answer is D (P, Q, R, and S). P is correct because mechanical digestion does occur in the stomach through the churning action of the stomach muscles. Q is correct as cholecystokinin, gastrin, and secretin are indeed produced by enteroendocrine cells in the gastrointestinal tract. R is correct because the pancreas, small intestine, and salivary glands produce digestive enzymes to aid in the breakdown of food. S is correct as mucous neck cells, chief cells, and parietal cells are all present in the stomach and play roles in the digestion process. Therefore, all options P, Q, R, and S are correct, making answer choice D the correct one.