Okay, let's tackle this question. The patient is a 15-year-old boy with leg pain when running short distances. His blood pressure is elevated at 165/90 mmHg, which is significant for his age. The radial pulses are strong (4+), but dorsalis pedis are weak (1+). Arterial blood gas is normal. The key here is the discrepancy in pulse strength between upper and lower extremities.

First, the elevated blood pressure and leg pain on exertion make me think of peripheral vascular issues. The weak pedal pulses suggest reduced blood flow to the legs. Since the radial pulses are normal, maybe there's a problem in the aorta or arteries that supply the lower body.

Coarctation of the aorta comes to mind. This is a congenital condition where there's a narrowing of the aorta, often after the subclavian arteries. This would explain why upper extremity BP is high (since the left subclavian is not affected) and lower extremity pulses are weak. The leg pain on exertion would be due to reduced blood flow during activity.

Other options: Tetralogy of Fallot can cause cyanosis but not pulse discrepancy. Patent ductus arteriosus usually presents with a continuous murmur and bounding pulses. Atrial septal defect might lead to right heart overload but not peripheral issues.

So, the correct answer is coarctation of the aorta. The clinical pearl here is the classic finding of upper extremity hypertension and lower extremity weak pulses, which is a hallmark of coarctation. Also, the leg pain on exertion fits with reduced perfusion during activity.

**Core Concept**
This question tests recognition of **coarctation of the aorta**, a congenital vascular anomaly causing hypertension in upper extremities and reduced lower extremity pulses. The key clinical clue is the **disproportionate upper vs. lower limb blood pressure** and **pulse weakness** in the legs.

**Why the Correct Answer is Right**
Coarctation of the aorta is a narrowing of the aorta, typically distal to the left subclavian artery. This creates high-pressure proximal to the coarctation (elevated upper limb BP) and reduced blood flow distal to the lesion (weak dorsalis pedis pulses). Leg pain on exertion (claudication) occurs due to ischemia during increased metabolic demand. The normal arterial blood gas rules out hypoxia, supporting a vascular rather than respiratory cause.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetralogy of Fallot causes cyanosis, not pulse discrepancies.
**Option B:** Patent ductus arteriosus presents with a continuous "machinery" murmur and bounding pulses.
**Option C:** Atrial septal defect leads to right heart volume overload but no peripheral pulse abnormalities.

**Clinical Pearl / High-Yield Fact**
Coarctation of the aorta often presents with **upper extremity hypertension** and **absent/weak femoral/dorsalis pedis pulses**. Remember the "3 Ds": **Dyspnea, Diaphoresis, and Dizziness** on exertion, plus **Delayed or absent femoral pulses**.

**Correct Answer: C. Coarctation of the aorta**

✓ Correct Answer: B. Coarctation of the aorta