Dark current in photoreceptors is due to:
Pick B: Opening of Na⁺ channels. Thinking about Sensory System Eye Physiology 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: cause and effect line up with Opening of Na⁺ channels — that is the mechanism or reason the stem is pointing to. Hence Opening of Na⁺ channels is correct. Treat this as a building block for the rest of Sensory System Eye Physiology; 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.
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.