What is the primary defect in myasthenia gravis?

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Deficiency of acetylcholine neurotransmitterReduction in acetylcholine receptor availabilityPresence of sustained muscle contractionsMuscle stiffness and hardnessExcessive release of acetylcholine

Explanation

Myasthenia gravis is characterized by a decrease in the number of functional acetylcholine receptors at the neuromuscular junction, leading to muscle weakness. It is not caused by a shortage of acetylcholine itself, nor by muscle rigidity or isometric contractions.