A 54-Year-Old Previously Healthy Man Comes to the Office Due

A 54-year-old previously healthy man comes to the office due to progressive fatigue and weakness for the past 2 months.  His weakness predominantly involves the lower extremities, causing him to have difficulty rising from a chair.  The patient also has a persistent cough and unintentional 7-kg (15.4-lb) weight loss during the same period.  He has a 30-pack-year smoking history and drinks alcohol occasionally.  Physical examination shows bilaterally decreased strength in the proximal muscles of the lower extremities with normal bulk and tone.  Quadriceps tendon reflexes are diminished bilaterally.  Chest x-ray reveals a hilar mass, biopsy of which demonstrates small cells that stain positive for chromogranin and synaptophysin.  Which of the following is the most likely cause of this patient's muscle weakness?

A) Antibodies against voltage-gated calcium channels
B) Antibodies against nicotinic acetylcholine receptors
C) Immune-mediated, segmental axonal demyelination
D) Neoplastic deposits in the spinal cord epidural space
E) Ubiquitin-proteasome-mediated muscle protein degradation

Correct Answer:

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