In Myasthenia gravis, the fundamental defect at the neuromuscular junction is:

Myasthenia gravis is the principal disease affecting the neuromuscular junction. In this process, the fundamental defect is not a deficiency of ACh or its release, but rather a failure of ACh to attach to the postsynaptic receptor, as a result of blockade by an antibody at the receptor site or of destruction of the membrane by antibody attack. There are several quite different synaptic disorders caused by botulism, aminoglycoside antibiotics, and the antibodies of the Lambe-Eaton myasthenic syndrome, which impede presynaptic release of ACh. Ref: Ropper A.H., Samuels M.A. (2009). Chapter 45. Electrophysiologic and Laboratory Aids in the Diagnosis of Neuromuscular Disease. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e.