A 63-Year-Old Man Comes to the Physician for the Evaluation

A 63-year-old man comes to the physician for the evaluation of difficulty walking for the last 6 months. He reports weakness in his lower legs that improves with exercise and throughout the day. Three months ago, he started adding flax seeds to his breakfast because he is frequently constipated. He has hypertension and type 2 diabetes mellitus. He has smoked 2 packs of cigarettes daily for the last 45 years. His current medications include enalapril and metformin. Vital signs are within normal limits. Examination shows dry mucous membranes. Muscle strength in the lower extremities is decreased. Sensation to pinprick and light touch is normal. Deep tendon reflexes are 1+ bilaterally. Active muscle contraction or repeated muscle tapping increases reflex activity. An x-ray of the chest shows an irregular, poorly demarcated mass at the right hilum.  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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