MCC Block 2 Practice Test 2026 - Free MCC Block 2 Practice Questions and Study Guide

Calcium in the blood is kept in balance mainly by parathyroid hormone (PTH), vitamin D, and calcitonin. PTH raises calcium when it’s low by signaling bone to release calcium, increasing kidney reabsorption of calcium, and promoting activation of vitamin D so the gut absorbs more calcium. If PTH production or action is reduced—hypoparathyroidism—these mechanisms don’t kick in, so serum calcium falls. That’s why hypoparathyroidism is a cause of hypocalcemia.

The other options don’t fit: excess PTH (hyperparathyroidism) raises calcium, not lowers it; high calcium (hypercalcemia) isn’t a cause of hypocalcemia; and excess vitamin D increases calcium absorption, also raising calcium rather than lowering it.