inhibiting secretion represents a long-loop negative feedback mechanism.

Correct Answer: A

Rationale: Cortisol inhibits hypothalamic CRH in a long-loop feedback.

Correct Answer: A

Rationale: An inactive thyroid has large follicles with abundant colloid and flat follicular cells due to low activity. Reabsorption lacunae (signs of hormone release) are absent. TSH would be elevated to stimulate the gland.

Correct Answer: D

Rationale: Conn's syndrome (primary hyperaldosteronism) causes K^+ depletion due to excess aldosterone. The medulla isn't essential, the cortex is ~90% of the gland, and 17α-hydroxylase is in zona fasciculata/reticularis.

Correct Answer: D

Rationale: Calcitonin secretion is stimulated by gastrin, CCK, secretin, and glucagon, all of which respond to high Ca2+ or digestive signals.

Correct Answer: B

Rationale: Thiamine deficiency causes beriberi (wet or dry), affecting nerves and heart; other options relate to different vitamin deficiencies.