Which of the following is an antagonist (nondepolarizing) neuromuscular blocker?

Cisatracurium is an antagonist (nondepolarizing) neuromuscular blocker that works by competitively inhibiting the action of acetylcholine at the neuromuscular junction. This inhibition reduces the ability of muscles to contract, leading to muscle relaxation, which is particularly useful during surgical procedures or for ventilation support.

Cisatracurium's mechanism of action is distinct from that of depolarizing agents like succinylcholine, which initially stimulate muscle contractions before causing paralysis. Other agents listed, such as ketamine, function primarily as anesthetics affecting pain perception and consciousness rather than directly blocking neuromuscular transmission. Atropine, an anticholinergic medication, primarily works by blocking the effects of acetylcholine on muscarinic receptors, impacting parasympathetic nervous system activity rather than serving as a neuromuscular blocker. Therefore, amongst the options provided, only cisatracurium fits the classification of a nondepolarizing neuromuscular blocker.