A 62-Year-Old Woman Has Double Vision, Especially When Reading at Bedtime

A 62-year-old woman has double vision, especially when reading at bedtime.  The problem first started several months ago and was intermittent but has worsened over the last 2 weeks and now bothers her constantly.  The patient has also developed difficulty chewing and swallowing foods.  She usually feels better in the mornings.  She has no extremity weakness or sensory symptoms.  The patient has no other medical problems and takes no medications.  She does not use tobacco, alcohol, or illicit drugs.  Vital signs are normal.  BMI is 27 kg/m².  Examination shows bilateral ptosis; there is weakness of adduction of the right eye and abduction of the left eye.  Muscle strength is 4/5 in proximal muscles in the upper and lower extremities.  Deep tendon reflexes are 2+ throughout. Which of the following changes involving the postsynaptic muscle cell most likely explains this patient's symptoms?

A) Impaired excitation-contraction coupling
B) Prolonged absolute refractory period
C) Reduced action potential amplitude
D) Reduced action potential propagation velocity
E) Reduced amplitude of motor end plate potential

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