Aldosterone is secreted by:

Correct Answer: C

Rationale: Aldosterone is secreted by the zona glomerulosa of the adrenal cortex.

Correct Answer: D

Rationale: Most amino acids are absorbed via Na^+-dependent transporters in the gut lumen; pepsinogens need acid, and carboxy peptidases are distinct enzymes.

Correct Answer: B

Rationale: Defective osteoclasts impair bone resorption, causing osteopetrosis (dense bones); other conditions involve different mechanisms.

Correct Answer: A

Rationale: Endocrine glands secrete directly into the bloodstream, whereas exocrine glands secrete via a duct.

Correct Answer: D

Rationale: The adrenal medulla secretes epinephrine and norepinephrine.