This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Post-tetanic potentiation is due to - 
Select and explain. 
A. Hyperpolarization of muscle fibres, B. Rapid K+ efflux, C. Increased availability of Ca++, D. Rapid Na+ influx
C: Increased availability of Ca++
Answer: C: Increased availability of Ca++ 
Explanation: Ans. is 'c' i.e., Increased availability of Ca++ "Repetitive stimulation enhances force development due to rise in intracellular Ca++, a phenomenon called posttetanic potentiation. It is due to increased phosphorylation of myosine light chain with increased number of cross bridges."