Which one of the following is absorbed in the GI tract without being digested?

Correct Answer: C

Rationale: The correct answer is C: vitamins. Vitamins are absorbed in the GI tract without being digested because they are already in a form that can be easily absorbed by the body. Unlike starch, proteins, and triglycerides which require digestion by enzymes to break them down into smaller molecules for absorption, vitamins are typically small molecules that can be directly absorbed into the bloodstream. Therefore, vitamins do not undergo digestion in the GI tract and are absorbed as they are. Starch, proteins, and triglycerides require digestion to be broken down into their respective components before they can be absorbed.

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: C

Rationale: The correct answer is C because duodenal cells release secretin in response to low pH in chyme. Secretin inhibits gastric secretions by stimulating the pancreas to release bicarbonate-rich fluid to neutralize the acidic chyme. Option A is incorrect because gastric inhibitory polypeptide and cholecystokinin actually stimulate pancreatic secretion, not inhibit gastric secretions. Option B is incorrect as the enterogastric reflex decreases gastric secretions to slow down the rate of emptying the stomach, not increase it. Option D is incorrect because enterogastrone is a hypothetical hormone and not involved in the regulation of gastric secretions.

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.