Identify functions of calcium within the body.

Correct Answer: D

Rationale: Calcium aids clotting (coagulation cascade), hardens bone (hydroxyapatite), and enables nerve conduction/muscle contraction (depolarization, release) multifaceted. Each is true: clotting stops bleeding, bone supports, nerve/muscle drive movement. All-inclusive role distinguishes calcium, key to physiology, unlike partial functions.

Correct Answer: A

Rationale: Melatonin (pineal) regulates sleep-wake cycles, peaking in darkness circadian driver. Water balance is ADH (pituitary), not melatonin. Sexual development ties to gonadal hormones (e.g., testosterone), not pineal puberty onset theories are minor. Sleep regulation distinguishes melatonin, critical for rhythm, unlike water or sex roles.

Correct Answer: C

Rationale: The thyroid, in front of the trachea, secretes T3/T4, setting basal metabolic rate (energy use) and aiding growth (via protein synthesis) neck-based. Brain hosts pituitary/hypothalamus (regulators), not direct producer. Bones lack glands growth target. Adrenals (kidneys) produce cortisol, not metabolism/growth primary. Thyroid's tracheal location distinguishes it, critical for metabolic and developmental regulation, unlike brain, bone, or adrenal sites.

Correct Answer: D

Rationale: Parathormone (PTH) from the parathyroid glands raises blood calcium by enhancing bone resorption, kidney reabsorption, and gut absorption via calcitriol not sodium, which aldosterone manages, nor does it lower calcium (calcitonin's role). This calcium-elevating action distinguishes PTH's homeostatic function, essential for nerve and muscle activity. Confusing sodium or calcium reduction with PTH misaligns its purpose, a common NEET trap highlighting endocrine specificity.