## **Core Concept**
Hypertensive crises in children, especially younger ones, are often secondary to an underlying disease process. Systemic hypertension in children can result from various causes, including renal diseases, cardiovascular issues, endocrine disorders, and more. Identifying the most common cause is crucial for diagnosis and management.

## **Why the Correct Answer is Right**
The correct answer, **C. Renal disease**, is supported by the fact that renal issues are the most common cause of secondary hypertension in children. Conditions such as **renal artery stenosis**, **glomerulonephritis**, and **chronic kidney disease** can lead to hypertension due to the kidney's role in regulating blood pressure through mechanisms like the renin-angiotensin-aldosterone system.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Cushing's syndrome** can cause hypertension due to excess cortisol, which has mineralocorticoid effects leading to sodium retention and hypertension, it is much rarer than renal causes.
- **Option B:** **Pheochromocytoma**, a tumor of the adrenal glands that leads to excess catecholamine release, can cause episodic hypertension but is relatively rare compared to renal disease.
- **Option D:** **Coarctation of the aorta**, a narrowing of the aortic isthmus, leads to hypertension in the arms but is less common than renal disease as a cause of systemic hypertension.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in children, especially those under 6 years old, **renal disease** is the most common cause of secondary hypertension. When evaluating a child with hypertension, a thorough investigation for underlying renal issues is essential.

## **Correct Answer:** . Renal disease