**Core Concept**
Nephrocalcinosis refers to the abnormal deposition of calcium salts within the kidney, often resulting from an imbalance in calcium metabolism or increased urinary calcium excretion. This condition can lead to renal calcification, which may cause various symptoms and complications.

**Why the Correct Answer is Right**
Medullary cystic kidney disease (MCKD) is a genetic disorder characterized by the progressive loss of kidney function due to the formation of cysts and fibrosis within the renal medulla. Unlike other conditions listed, MCKD does not typically lead to nephrocalcinosis. The primary pathophysiology of MCKD involves mutations in genes that regulate cellular transport and ion balance, rather than calcium metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** Sarcoidosis is a systemic granulomatous disease that can cause hypercalcemia due to increased 1,25-dihydroxyvitamin D production by granulomatous tissues, leading to nephrocalcinosis.
**Option C:** Milk-alkali syndrome is characterized by hypercalcemia, metabolic alkalosis, and renal calcification, resulting from excessive calcium and absorbable alkali intake.
**Option B:** Distal renal tubular acidosis (A) can cause nephrocalcinosis due to impaired acid excretion, leading to increased urinary calcium excretion and calcification of the renal medulla.

**Clinical Pearl / High-Yield Fact**
Nephrocalcinosis can be a sign of underlying systemic or metabolic disorders, emphasizing the importance of a comprehensive evaluation of patients with renal calcification.

**✓ Correct Answer: D. Medullary cystic kidney.**