A 42-Year-Old Woman Comes to the Physician Because of Progressive

A 42-year-old woman comes to the physician because of progressive weakness for the past month. She has noticed increasing difficulty performing household chores and walking her dog. Sometimes, she feels too fatigued to cook dinner. She has noticed that she feels better after sleeping. She does not have chest pain, shortness of breath, or a history of recent illness. She has no other history of serious illness and takes no medications. She has smoked two packs of cigarettes daily for 25 years. She appears fatigued. Examination shows drooping of the upper eyelids. There is diminished motor strength in the upper extremities. Sensation and reflexes are intact. 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