**Core Concept**
Polycystic kidney disease (PKD) is a genetic disorder characterized by the development of numerous cysts in the kidneys, leading to renal tubular dilatation and impaired kidney function. The cysts are typically filled with fluid and can vary in size, causing distortion of the normal kidney architecture.

**Why the Correct Answer is Right**
In PKD, the cysts are formed due to the abnormal proliferation of renal epithelial cells, which leads to the dilatation of renal tubules. The cysts are lined by a layer of epithelial cells and are filled with fluid, leading to the characteristic cylindrical dilatation of renal tubules. The cysts can be found in both kidneys and can vary in size and number, leading to progressive renal impairment.

**Why Each Wrong Option is Incorrect**
**Option A:** Medullary cystic disease is a separate genetic disorder characterized by the formation of cysts in the renal medulla, but it does not typically cause cylindrical dilatation of renal tubules.

**Option B:** Wilms tumor is a type of kidney cancer that affects children, but it does not cause cystic dilatation of renal tubules.

**Option C:** Lipoid nephrosis, also known as minimal change disease, is a type of glomerulonephritis that affects the glomeruli, but it does not cause cystic dilatation of renal tubules.

**Clinical Pearl / High-Yield Fact**
Polycystic kidney disease is a genetic disorder that can be inherited in an autosomal dominant or autosomal recessive pattern, and it is one of the most common genetic disorders affecting the kidneys.

✓ Correct Answer: A. Polycystic disease of kidney