**Core Concept**
The primary site of Na+ reabsorption by the Na+-H+ exchange mechanism in the nephron is the proximal convoluted tubule (PCT). This process plays a crucial role in maintaining acid-base balance and regulating sodium levels in the body.

**Why the Correct Answer is Right**
In the PCT, the sodium-hydrogen exchanger (NHE3) is the primary transporter responsible for Na+ reabsorption by exchanging Na+ ions for H+ ions. This mechanism helps to reclaim approximately 65-70% of filtered sodium from the glomerular filtrate, which is essential for maintaining proper sodium levels and preventing sodium wasting. The Na+-H+ exchange mechanism is driven by the electrochemical gradient generated by the basolateral Na+/K+-ATPase pump, which pumps sodium out of the cell into the interstitium.

**Why Each Wrong Option is Incorrect**
**Option B:** The loop of Henle, particularly the thick ascending limb, is primarily involved in Na+ reabsorption through the Na+-K+-2Cl- cotransporter (NKCC2), but not by the Na+-H+ exchange mechanism.

**Option C:** The distal convoluted tubule (DCT) is involved in Na+ reabsorption, but it primarily uses the thiazide-sensitive Na+-Cl- cotransporter (NCC), not the Na+-H+ exchange mechanism.

**Option D:** The collecting duct is involved in fine-tuning sodium reabsorption, but it primarily uses the epithelial sodium channel (ENaC) and the aldosterone-sensitive Na+-K+-ATPase pump, not the Na+-H+ exchange mechanism.

**Clinical Pearl / High-Yield Fact**
The Na+-H+ exchange mechanism in the PCT is highly regulated by various factors, including pH, sodium delivery, and parathyroid hormone (PTH) levels. A defect in this mechanism can lead to conditions such as proximal renal tubular acidosis (RTA), characterized by metabolic acidosis, hypokalemia, and hyperchloremia.

**✓ Correct Answer: A. PCT**