Which statement best describes the pathophysiology of myasthenia gravis?

• A. Autoimmune destruction of acetylcholine receptors at the neuromuscular junction
• B. Defective release of acetylcholine from motor neurons due to autoimmune attack on the vesicles
• C. Congenital deficiency of acetylcholine
• D. Primary muscle fiber degeneration

Answer: (A)

The pathophysiology of myasthenia gravis involves an autoimmune attack on the neuromuscular junction that targets the postsynaptic acetylcholine receptors. Autoantibodies (most commonly IgG) bind to nicotinic ACh receptors on the motor endplate, leading to receptor cross-linking, internalization, and complement-mediated damage. This reduces the number of functional receptors available to respond to acetylcholine, so the end-plate potential from each neural impulse may fail to reach the threshold needed to trigger a muscle action potential. The result is fatigable, fluctuating muscle weakness that worsens with activity and improves with rest. Treatments that boost acetylcholine at the junction (like acetylcholinesterase inhibitors) help compensate for the reduced receptor availability, and thymic abnormalities often drive the autoantibody production.

Other scenarios don’t fit MG as well: a presynaptic release defect would point to a different condition (e.g., Lambert-Eaton syndrome), congenital deficiencies of acetylcholine describe congenital myasthenic syndromes, and primary muscle fiber degeneration describes a myopathy, not a junctional transmission problem.