Identify a common neuromuscular disorder and briefly describe its mechanism.

• A. Myasthenia gravis — autoimmune antibodies target acetylcholine receptors at the NMJ, reducing signaling and causing fatigable weakness.
• B. Duchenne muscular dystrophy — X-linked dystrophin gene mutation leading to membrane instability.
• C. Amyotrophic lateral sclerosis — motor neuron degeneration with muscle atrophy.
• D. Lambert-Eaton syndrome — antibodies block voltage-gated calcium channels in presynaptic terminals.

Answer: (A)

A common neuromuscular disorder is myasthenia gravis, which highlights how autoimmune disruption at the neuromuscular junction impairs signaling. In this condition, antibodies target nicotinic acetylcholine receptors on the motor end plate, reducing the number of receptors available for acetylcholine to bind. With fewer functional receptors, the end-plate potential may not reach the threshold needed to trigger a muscle action potential, especially after repeated use, so muscles become progressively weaker with activity and improve with rest. This contrasts with Duchenne muscular dystrophy, where membrane instability arises from a dystrophin mutation; amyotrophic lateral sclerosis, which involves motor neuron degeneration; and Lambert-Eaton syndrome, where antibodies block presynaptic voltage-gated calcium channels and decrease acetylcholine release.