Which of the following is a cause of hypocalcemia?

• A. Hyperparathyroidism
• B. Milk-alkali syndrome
• C. Vitamin D deficiency
• D. Hypercalcemia

Answer: (C)

Calcium balance hinges on intestinal absorption, bone turnover, and renal handling, all driven by vitamin D. When vitamin D is deficient, the gut cannot absorb calcium well, so serum calcium falls. The parathyroid gland responds by increasing PTH to raise calcium from bone and boost renal reabsorption, but without vitamin D there isn’t enough intestinal absorption to fully correct the deficit, so hypocalcemia persists.

Hyperparathyroidism raises calcium levels, not lowers it. Milk-alkali syndrome leads to hypercalcemia. Hypercalcemia itself refers to high calcium, not low, so those scenarios don’t cause hypocalcemia.