This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anaesthesia, Subject name: Pharmacology
Following drug inhibits release of calcium from sarcoplasmic reticulum - 
Select and explain. 
A. Dantrolene, B. Caffiene, C. Rocuronium, D. Succnylcholine
A: Dantrolene
Answer: A: Dantrolene 
Explanation: Ans. is 'a' i.e., Dantrolene Dantrolene* This muscle relaxant is chemically and pharmacologically entirely different from neuromuscular blockers; effect superficially resembles that of centrally acting muscle relaxants.* Neuromuscular transmission or MAP are not affected, but muscle contraction is uncoupled from depolarization of the membrane.* Dantrolene acts on the RyRl (Ryanodine Receptor) calcium channels in the sarcoplasmic reticulum of skeletal muscles and prevents Ca2+ induced Ca2+ release through these channels.* Intracellular release of Ca2+ needed for excitation-contraction coupling is interfered with.* Fast contracting 'twitch* muscles are affected more than slow contracting 'antigravity* muscles.* Since Ca2+ channels in the sarcoplasmic reticulum of cardiac and smooth muscles are of a different subtype (RyR2), these muscles are affected little by dantrolene.