This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nerve Physiology, Subject name: Physiology
Hyperpolarization is caused by which ions - 
Select and explain. 
A. K+, B. Na+, C. Ca-+, D. HCO3
A: K+
Answer: A: K+ 
Explanation: Ans. is ' a' i.e., K+ o During depolarization (spike potential) there is opening of Na+ channels causing Na~ influx. This causes membrane potential to become +35 to +40 mV (from -70 mV of resting potential),o During repolarization phase channels open and there is K+ efflux. The membrane voltage falls back (repolarizes) to resting potential, i.e., -70 mV. At this voltage. K+ channels close, but quite slowly. Therefore, more K+ ions leak out, causing a brief period in which the voltage falls below' -70 mV, i.e. hyperplorization.o Then, K+/Na~ pump works to re-establish resting potential by pumping N+ ions back out and K+.' ions back into the cells.