A 20-year-old woman complains of weakness that is worse in the afternoon, worse during prolonged activity, and improved by rest. When fatigued, the patient is unable to hold her head up or chew her food. She often notes diplopia when driving home from work. On physical examination, she has no loss of reflexes, sensation, or coordination. Which of the following is the likely pathogenesis of this disease?
A 20-year-old woman complains of weakness that is worse in the afternoon, worse during prolonged activity, and improved by rest. When fatigued, the patient is unable to hold her head up or chew her food. She often notes diplopia when driving home from work. On physical examination, she has no loss of reflexes, sensation, or coordination. Which of the following is the likely pathogenesis of this disease?
💡 Explanation
**Core Concept**
Myasthenia gravis (MG) is an autoimmune disease characterized by weakness and fatigue of skeletal muscles, which worsens with activity and improves with rest. The disease is caused by autoantibodies targeting the acetylcholine receptor (AChR) at the neuromuscular junction, leading to impaired neuromuscular transmission.
**Why the Correct Answer is Right**
In MG, autoantibodies directed against the postsynaptic AChR cause a reduction in the number of functional AChR, leading to decreased acetylcholine binding and subsequent neuromuscular transmission failure. This results in muscle weakness and fatigue, which worsen with activity and improve with rest. The patient's symptoms, such as diplopia, difficulty swallowing, and weakness, are classic manifestations of MG. The absence of sensory or motor deficits, such as loss of reflexes, sensation, or coordination, further supports the diagnosis of MG.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to the pathogenesis of myasthenic syndrome, which is caused by antibodies against the muscle-specific kinase (MuSK), not the AChR.
**Option B:** This option is incorrect because destruction of anterior horn cells by virus is characteristic of poliomyelitis, not MG.
**Option C:** This option is incorrect because progressive muscular atrophy caused by spinal degeneration is characteristic of amyotrophic lateral sclerosis (ALS), not MG.
**Option D:** This option is incorrect because demyelinating disease, such as multiple sclerosis, is characterized by sensory and motor deficits, which are not present in MG.
**Clinical Pearl / High-Yield Fact**
MG is often associated with thymic abnormalities, including thymoma, and may require thymectomy as part of treatment. Additionally, MG can be triggered or exacerbated by certain medications, such as aminoglycoside antibiotics and beta-blockers.
**✓ Correct Answer: A. Autoantibodies directed against the postsynaptic acetylcholine receptor causing neuromuscular transmission failure**
✓ Correct Answer: A. Autoantibodies directed against the postsynaptic acetylcholine receptor causing neuromuscular transmission failure
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