Nodal cell depolarization is mainly due to influx of:
Correct option is C, Ca2+. In Cardiovascular System: Heart, the accurate description is Ca2+, and that is what you should commit to memory. Keep the definition tight and the role clear — the functional job described in the stem is exactly what Ca2+ does in Cardiovascular System: Heart. Then Ca2+ is the answer you can defend. Check yourself by covering the options and writing the answer first, then matching the letter — that builds confidence. If the wording feels dense, translate it into everyday language first, then map that plain sentence back onto Ca2+. Remember that physiology questions reward precise language: name the process, the location, and the outcome, then match that to the option text.
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.