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: Unknown
[{'content': 'd-Tubocurarine -Mechanism of action: \nSelect and explain. \nA. Competitive, nondepolarizing block at the Nm cholinergic receptors, B. Noncompetitive, nondepolarizing block at the Nm cholinergic receptors, C. Competitive, depolarizing block at the Nm cholinergic receptors, D. Noncompetitive, Depolarizing block at the Nm cholinergic receptors', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ref. KDT. Page. 352\nd-TC\n\nThe competitive blockers have affinity for the nicotinic (NM) cholinergic receptors at the muscle end plate, but have no intrinsic activity\nMost of the competitive blockers have two or more quaternary N+ atoms which provide the necessary attraction to the same site, but the bulk of the antagonist molecule does not allow conformational changes in the subunits needed for opening the channel\nd-TC thus reduces the frequency of channel opening but not its duration or the conductance of a channel once it has opened.\nThe competitive blockers also block prejunc- tional nicotinic receptors located on motor nerve endings', 'role': 'assistant'}]