In the kidneys, parathyroid hormone causes:

Correct Answer: A

Rationale: Parathyroid hormone (PTH), secreted by the parathyroid glands when blood calcium drops, regulates calcium and phosphate balance. In the kidneys, PTH enhances calcium reabsorption in the distal tubules, reducing urinary calcium loss and raising blood levels. Concurrently, it inhibits phosphate reabsorption in the proximal tubules, increasing phosphate excretion and lowering blood phosphate. This dual action maintains mineral homeostasis elevated calcium supports bone and muscle function, while reduced phosphate prevents soft tissue calcification. The kidneys filter 180 liters of plasma daily, and PTH fine-tunes reabsorption to counter hypocalcemia, often triggered by dietary or hormonal shifts. Calcium loss with phosphate retention would worsen hypocalcemia, contradicting PTH's purpose. Losing both minerals isn't selective enough for balance, and retaining both would raise phosphate, risking imbalances. PTH also mobilizes calcium from bone and boosts vitamin D activation, but its renal effect is immediate and precise. Endocrine physiology texts confirm this mechanism, highlighting PTH's critical role in mineral metabolism, distinct from other hormones like calcitonin, which lowers calcium.

Correct Answer: B

Rationale: Melanin, produced by melanocytes in the stratum basale, determines skin color, ranging from light to dark, and protects against UV by absorbing it. Hemoglobin colors blood, visible through skin but not primary. Carotene adds yellowish tones, minorly. Keratin is a structural protein, not a pigment. Melanin's dominant role in pigmentation and photoprotection, per dermatology, makes 'b' correct.

Correct Answer: C

Rationale: Osteoclasts resorb bone, breaking down matrix via enzymes/acids to release calcium, balancing remodeling. Osteoblasts form bone, osteocytes maintain it, and chondrocytes produce cartilage, not bone resorbers. Osteoclasts' destructive role, per bone physiology, makes 'c' correct.

Correct Answer: D

Rationale: The vertebral column protects the spinal cord (not heart, a), supports the head , and allows movement/flexibility via joints e.g., cervical flexion. Heart protection is rib cage/sternum. All functions apply, per spinal anatomy, making 'd' correct.

Correct Answer: A

Rationale: Five lumbar vertebrae (L1-L5) form the lower back, supporting weight and flexibility. Seven is cervical, twelve is thoracic, three is miscount. Lumbar's count, per spinal anatomy, makes 'a' correct.