**Core Concept**
Medullary cystic disease of the kidney (MCDK) is a rare congenital disorder characterized by the formation of multiple cysts in the medulla of the kidney, often leading to renal failure. The diagnosis of MCDK typically involves a combination of imaging studies and histopathological examination.

**Why the Correct Answer is Right**
The correct answer is **B. Renal biopsy**, which allows for histopathological examination of the kidney tissue. This is the most definitive method for diagnosing MCDK, as it enables the identification of the characteristic cysts and fibrosis in the medulla. Renal biopsy can also help differentiate MCDK from other kidney diseases that may present with similar symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Imaging studies such as ultrasound or CT scans can suggest the presence of MCDK, but they are not definitive in diagnosing the disease. Imaging studies may not always detect the characteristic cysts in the medulla.
**Option C:** Serum creatinine levels may be elevated in patients with MCDK, but this is a non-specific finding and does not confirm the diagnosis. Elevated serum creatinine can be seen in many other kidney diseases.

**Clinical Pearl / High-Yield Fact**
MCDK is often associated with other congenital anomalies, such as polycystic kidney disease or von Hippel-Lindau syndrome, and may also be seen in patients with tuberous sclerosis complex.

**Correct Answer: B. Renal biopsy**