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
When a muscle is not contracting, actin and myosin are prevented from reacting by: 
Select and explain. 
A. Troponin-tropomyosin, B. Phosphocreatine, C. Heavy meromyosin, D. Acetylcholinesterase
A: Troponin-tropomyosin
Answer: A: Troponin-tropomyosin 
Explanation: When a muscle is not contracting, actin and myosin are prevented from reacting by troponin-tropomyosin. Tropomyosin covers the active sites on actin filaments which prevents myosin heads from attachment. Troponin must bind with Ca++ in order to cause the tropomyosin to move off the active sites. Phosphocreatine has a high energy phosphate bond which is used to reconstitute ATP. Heavy meromyosin refers to the heavy chains in the myosin filament. Acetylcholinesterase and acetylcholine are at the neuromuscular junctions and do not directly affect actin and myosin crossbridge formation. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong's Review of Medical Physiology, 24e.