**Core Concept**
Hypercalciuria refers to the excessive excretion of calcium in the urine, often caused by increased absorption of calcium from the gut, increased bone resorption, or decreased renal calcium reabsorption. It can lead to kidney stones and other complications.

**Why the Correct Answer is Right**
The treatment of hypercalciuria aims to reduce urinary calcium excretion. Thiazide diuretics, such as chlorothiazide, are effective in reducing calcium excretion by increasing calcium reabsorption in the distal convoluted tubule of the kidneys. This is achieved through the inhibition of the sodium-chloride cotransporter, which leads to increased intracellular calcium levels and subsequent reduced calcium excretion.

**Why Each Wrong Option is Incorrect**
**Option A:** Furosemide is a loop diuretic that increases calcium excretion, making it an incorrect choice for treating hypercalciuria.
**Option B:** Hydrochlorothiazide is the correct answer, and option B is likely a distractor with a similar name.
**Option C:** Allopurinol is a xanthine oxidase inhibitor used to treat gout and hyperuricemia, not hypercalciuria.

**Clinical Pearl / High-Yield Fact**
Thiazide diuretics can also be used to treat calcium nephrolithiasis by reducing urinary calcium excretion and increasing calcium reabsorption in the kidneys.

**Correct Answer: C. Hydrochlorothiazide**