The middle portion of the small intestine is the

Correct Answer: B

Rationale: The jejunum is the middle section.

Correct Answer: C

Rationale: The correct answer is C: jejunum. The majority of nutrient absorption occurs in the jejunum due to its extensive surface area, lined with villi and microvilli, which increase absorption efficiency. Digestive enzymes and bile also aid in breaking down nutrients in the jejunum. The ascending colon (A) primarily absorbs water and electrolytes, not nutrients. The stomach (B) mainly digests food through acid and enzymes but absorbs only a small amount of certain substances like alcohol and aspirin. The ileum (D) absorbs bile salts, vitamin B12, and some electrolytes, but nutrient absorption is most efficient in the jejunum.

Correct Answer: D

Rationale: The correct answer is D: villi. Villi are finger-like projections in the small intestine that greatly increase the surface area for nutrient absorption. They have microvilli on their surface, further enhancing absorption efficiency. Circular folds (A) are also known as plicae circulares and increase surface area for absorption, but they are not finger-like projections. Cilia (B) are hair-like structures involved in movement and are not found in the small intestine. Haustra (C) are pouches in the large intestine that aid in the mixing and propulsion of feces, not in the small intestine's absorption process.

Correct Answer: D

Rationale: The correct answer is D: secretin. Secretin is a hormone released by the duodenum in response to the acidic chyme entering from the stomach. Secretin stimulates the pancreas to release its bicarbonate-rich pancreatic juice to neutralize the acidic chyme. This helps create an optimal pH environment for pancreatic enzymes to function effectively. HCl in chyme (A) is actually the stimulus for secretin release. Trypsinogen (B) is an inactive form of trypsin and is not involved in stimulating pancreatic secretions. Insulin (C) is a hormone produced by the pancreas that regulates blood sugar levels and is not directly involved in stimulating pancreatic secretions.

Correct Answer: B

Rationale: The correct answer is B because the stomach's primary function is to store food, churn it to mix it with digestive enzymes, and begin the process of digestion. This is essential for breaking down food into smaller particles to facilitate nutrient absorption in the small intestine. Choice A is incorrect as the main organ for fat digestion is the small intestine. Choice C is incorrect as nutrient absorption primarily occurs in the small intestine. Choice D is incorrect as packaging feces is the role of the large intestine, not the stomach.