**Core Concept**
The hydrocortisone suppression test is used to differentiate between hypercalcemia of malignancy and primary hyperparathyroidism by assessing the effect of glucocorticoids on calcium levels. This test is based on the fact that glucocorticoids can suppress the production of parathyroid hormone (PTH) in primary hyperparathyroidism, whereas in malignancy-induced hypercalcemia, PTH production is not suppressed.

**Why the Correct Answer is Right**
In primary hyperparathyroidism, the parathyroid glands produce excess PTH, leading to hypercalcemia. Glucocorticoids can suppress the production of PTH in these glands, resulting in a decrease in calcium levels. In contrast, hypercalcemia of malignancy is often due to the production of PTH-related protein (PTHrP) by tumor cells, which is not suppressed by glucocorticoids. Therefore, a positive response to hydrocortisone suppression is indicative of primary hyperparathyroidism, while a lack of response suggests hypercalcemia of malignancy.

**Why Each Wrong Option is Incorrect**

* **Option B:** The glucose challenge test is used to diagnose diabetes mellitus, not to differentiate between hypercalcemia of malignancy and primary hyperparathyroidism.
* **Option C:** 24-hour urine calcium measurement is used to assess calcium excretion and may be helpful in diagnosing hyperparathyroidism, but it is not specific for differentiating between the two conditions.
* **Option D:** Percentage of ionized calcium is a measure of the biologically active form of calcium in the blood, but it does not provide information about the underlying cause of hypercalcemia.

**Clinical Pearl / High-Yield Fact**
The hydrocortisone suppression test is a useful tool in differentiating between primary hyperparathyroidism and hypercalcemia of malignancy, but it should be interpreted in conjunction with other clinical and laboratory findings.

**✓ Correct Answer: A. Hydrocortisone suppression test**