**Core Concept**
The question is testing the normal variation in kidney size and the threshold at which a significant difference raises suspicion for unilateral kidney disease. Normal kidney size can vary between individuals, but a significant difference in size between the two kidneys may indicate a pathological process.

**Why the Correct Answer is Right**
A difference of more than 2 cm in the size of two kidneys raises suspicion for unilateral kidney disease. This is because the normal variation in kidney size between individuals is generally less than 1.5 cm. A difference of more than 2 cm suggests a pathological process such as kidney disease, tumor, or other abnormalities that may affect the size of one kidney. This is because the kidneys are paired organs, and a significant difference in size between them may indicate an underlying problem.

**Why Each Wrong Option is Incorrect**
**Option A:** A difference of less than 1 cm is generally considered within the normal range, and a difference of this magnitude is unlikely to raise suspicion for unilateral kidney disease.

**Option B:** A difference of 1-1.5 cm may be considered borderline, but it is not typically sufficient to raise strong suspicion for unilateral kidney disease. Clinical judgment and additional diagnostic tests are usually necessary to determine the cause of a small difference in kidney size.

**Option C:** A difference of more than 2 cm is correct, but the question asks for the threshold at which a significant difference raises suspicion for unilateral kidney disease, not the correct answer.

**Option D:** A difference of less than 0.5 cm is generally not significant and is unlikely to raise suspicion for unilateral kidney disease.

**Clinical Pearl / High-Yield Fact**
When evaluating kidney size, it's essential to consider the patient's overall clinical context, including symptoms, laboratory results, and imaging studies. A significant difference in kidney size may be an important clue to underlying kidney disease, but it should be interpreted in conjunction with other clinical findings.

**Correct Answer:** C.