Read the following statements A and B and choose the correct option w.r.t. nephron in human kidney.

Statement A: Potassium ions are reabsorbed as well as secreted in PCT.

Statement B: Countercurrent mechanism helps to maintain a concentration gradient in the medullary interstitium by increasing absorption of urea from DCT.

Solution:

Statement A: In the Proximal Convoluted Tubule (PCT), approximately 65-70% of filtered potassium ions (K+) are reabsorbed. While the major site for regulated K+ secretion is the Distal Convoluted Tubule (DCT) and collecting duct, some minor secretion of K+ can also occur in the PCT. Therefore, the statement that K+ ions are reabsorbed as well as secreted in PCT is considered correct.

Statement B: The countercurrent mechanism, involving the Loop of Henle and vasa recta, is essential for maintaining a high concentration gradient in the medullary interstitium, which is crucial for concentrating urine. Urea contributes significantly to this gradient. However, urea is primarily reabsorbed from the inner medullary collecting duct into the medullary interstitium, not from the Distal Convoluted Tubule (DCT). The DCT is mainly involved in conditional reabsorption of Na+ and water, and secretion of K+ and H+. Therefore, statement B is incorrect.

Based on the analysis, Statement A is correct and Statement B is incorrect. Thus, option (4) is the correct answer.