## **Core Concept**
The question pertains to the clinical presentation and pathological findings associated with unilateral kidney abnormalities, specifically a granular contracted kidney on one side and a normal kidney on the other. This scenario can arise from various renal pathologies that lead to asymmetrical kidney involvement.

## **Why the Correct Answer is Right**
The correct answer, **C. Chronic pyelonephritis**, is right because chronic pyelonephritis can cause unilateral or asymmetrical involvement of the kidneys. This condition results from recurrent or persistent infections leading to inflammation and scarring in the kidneys. Over time, the affected kidney may become contracted and granular due to fibrosis and loss of renal parenchyma, while the other kidney remains normal if it is not affected.

## **Why Each Wrong Option is Incorrect**
- **Option A:** **Polycystic kidney disease** typically presents bilaterally, with multiple cysts in both kidneys, leading to their enlargement. It is less likely to present unilaterally with one kidney being normal.
- **Option B:** **Renal artery stenosis** can lead to atrophy and shrinkage of the affected kidney due to decreased blood flow, but it usually doesn't cause a granular appearance; rather, it results in a smaller, smoother kidney.
- **Option D:** **Glomerulonephritis** usually affects both kidneys simultaneously and presents with different clinical and pathological features, such as hematuria, proteinuria, and specific glomerular changes on biopsy.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that chronic pyelonephritis can cause scarring and contraction of the affected kidney, leading to a characteristic "salt and pepper" appearance on imaging due to the mixture of fibrosis and atrophy. This condition often results from recurrent urinary tract infections and can lead to significant morbidity if not properly managed.

## **Correct Answer:** C. Chronic pyelonephritis