A patient had elevated calcium in the blood. What is the diagnosis?

HYPERCALCEMIA (elevated blood calcium) is caused by HYPERPARATHYROIDISM (excess PTH → increases calcium resorption from bone, increases renal calcium reabsorption, increases intestinal absorption). Normal serum calcium: 8.5–10.4 mg/dL. Values above this indicate hypercalcemia. HYPOPARATHYROIDISM causes LOW calcium (hypocalcemia — A wrong). Vitamin D deficiency causes LOW calcium (B wrong). Hyperparathyroidism is the classic cause of hypercalcemia (along with malignancy). C is correct.