**Core Concept**
Sustained severe hypertension in children can be caused by various factors, but the commonest cause is often related to renal abnormalities. The kidneys play a crucial role in regulating blood pressure, and any impairment in kidney function can lead to elevated blood pressure.

**Why the Correct Answer is Right**
The correct answer is related to renal causes of hypertension, specifically **Congenital Adrenal Hyperplasia (CAH)**, which is not the answer but we will get to it in the distractors. However, the commonest cause is often related to **Renal Parenchymal Disease**, such as **Chronic Pyelonephritis** or **Nephritis**. These conditions can cause scarring and fibrosis in the kidneys, leading to impaired kidney function and subsequently elevated blood pressure. The renin-angiotensin-aldosterone system (RAAS) is often activated in these conditions, further contributing to hypertension.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Congenital Adrenal Hyperplasia (CAH)** is a rare cause of hypertension in children and is not the commonest cause.
* **Option B:** **Coarctation of the Aorta** is a congenital heart defect that can cause hypertension, but it is not the most common cause in children.
* **Option C:** **Essential Hypertension** is a common cause of hypertension in adults, but it is not typically the primary cause in children.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hypertension in children can be a sign of an underlying renal or cardiovascular condition, and a thorough investigation is necessary to identify the underlying cause.

**Correct Answer:** D.