Remember:
"Facultative" always points to the Distal Convoluted Tubule (DCT), as it is regulated by hormones (aldosterone and ADH).
"Descending limb" is always permeable to water, never to electrolytes.
Step 1: Understanding the Question:
The task is to match different segments of the nephron (List-I) with their specific transport and regulatory functions (List-II).
Step 2: Key Concept and Approach:
We will evaluate the standard physiological properties of the nephron segments:
1. Proximal Convoluted Tubule (PCT) selectively secretes hydrogen and ammonia to maintain acid-base balance.
2. Descending limb of Henle's loop is permeable only to water, allowing selective water reabsorption.
3. Distal Convoluted Tubule (DCT) is responsible for facultative (hormonally regulated) reabsorption of sodium and water.
4. The Collecting Duct plays a role in the selective secretion of $H^+$ and $K^+$ ions to maintain blood pH and ionic concentration.
Step 3: Detailed Explanation:
• A. Proximal convoluted tubule -- III. Selective secretion of $H^+$, $NH_3$: PCT helps to maintain pH by secreting hydrogen ions, ammonia, and potassium ions into the filtrate.
• B. Collecting duct -- I. Selective secretion of $H^+$, $K^+$: Collecting duct maintains ionic and pH balance of blood by selective secretion of $H^+$ and $K^+$ ions.
• C. Descending limb of loop of Henle -- IV. Selective reabsorption of $H_2O$: This segment is highly permeable to water but impermeable to electrolytes, leading to concentration of the filtrate.
• D. Distal convoluted tubule -- II. Facultative reabsorption of $Na^+$, $H_2O$: Reabsorption of water and sodium here is conditional, regulated by ADH and aldosterone.
• This gives the match: A-III, B-I, C-IV, D-II.
Step 4: Final Answer:
The correct matching sequence is A-III, B-I, C-IV, D-II, which corresponds to Option (C).
