Which is TRUE?

Correct Answer: B

Rationale: T_3 (triiodothyronine) is more potent than T_4 (thyroxine), though T_4 is secreted in larger amounts and converted to T_3 peripherally. The thyroid is not essential for life (adults can survive thyroidectomy with hormone replacement), RT_3 is inactive, and T_3 does not convert to T_4.

Correct Answer: C

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

Correct Answer: D

Rationale: Prostaglandin E inhibits hormone-sensitive lipase, reducing lipolysis, while adrenaline, thyroxine, and TSH stimulate it; serotonin has minimal direct effect.

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.