Na+ reabsorption is maximum in –
**Core Concept**
The kidney plays a crucial role in maintaining electrolyte balance through the reabsorption and secretion of ions, including sodium (Na+). The renal tubules are responsible for the majority of Na+ reabsorption, which occurs mainly in the proximal convoluted tubule (PCT) and the thick ascending limb of the loop of Henle.
**Why the Correct Answer is Right**
The PCT is the site of maximum Na+ reabsorption due to the presence of the sodium-potassium pump (Na+/K+ ATPase) and the sodium-glucose cotransporter 2 (SGLT2). As glucose and amino acids are reabsorbed from the filtrate in the PCT, sodium ions follow passively due to their electrochemical gradient. This process helps maintain the electroneutrality of the filtrate and prevents excessive loss of sodium in the urine.
**Why Each Wrong Option is Incorrect**
**Option A:** The descending limb of the loop of Henle is primarily involved in water reabsorption, not sodium reabsorption.
**Option B:** The distal convoluted tubule is responsible for fine-tuning sodium reabsorption and potassium secretion, but it is not the site of maximum sodium reabsorption.
**Option C:** The collecting duct is involved in the regulation of sodium reabsorption and potassium secretion, but it is not the primary site of sodium reabsorption.
**Clinical Pearl / High-Yield Fact**
The kidneys reabsorb approximately 99% of filtered sodium, which is essential for maintaining blood pressure and electrolyte balance. Impaired sodium reabsorption can lead to hyponatremia and hypovolemia.
**Correct Answer:** A. Proximal convoluted tubule (PCT)