Na⁺–H⁺ antiporter in PCT is important for reabsorption of:
Pick B: Bicarbonate. Thinking about Excretory System in a practical way — what the structure does, or what the process achieves — leads you here. Clinically and academically, the same logic shows up again and again: Bicarbonate is the option that correctly names the structure, process, or principle asked for in Excretory System. Hence Bicarbonate is correct. Treat this as a building block for the rest of Excretory System; neighbouring topics often reuse the same principle. Keep a one-line summary card for this idea and revisit it before the paper; short, repeated review beats long cramming sessions. If the wording feels dense, translate it into everyday language first, then map that plain sentence back onto Bicarbonate.
Ref: Animal physiology is Unit 7 of the CSIR NET Life Science Syllabus, covering core body systems. It accounts for roughly 10% of the total marks in Sections B and C.