Regarding calcium metabolism:

Correct Answer: D

Rationale: Chronic low Ca2+ triggers secondary hyperparathyroidism. High Ca2+ suppresses PTH and doesn't cause tetany (low Ca2+ does). ~99% of Ca2+ is in bones, and infant turnover is high but not precisely 80%.

Correct Answer: B

Rationale: Folate is absorbed via a proton-coupled transporter, not Na^+-dependent like thiamine, niacin, riboflavin, and pyridoxine.

Correct Answer: C

Rationale: VIP, GIP, and glucagon do not stimulate gastrin; atropine doesn't inhibit it, amino acids do stimulate, secretin inhibits, and epinephrine can increase it.

Correct Answer: A

Rationale: The esophagus lacks BER, relying on peristalsis triggered by swallowing, unlike other GIT segments.

Correct Answer: C

Rationale: The actual structural unit that secretes insulin is the beta cell. Beta cells are found on the islets of Langerhans, which are in the pancreas.