A 45-Year-Old Previously Healthy Man Comes to the Office Due

A 45-year-old previously healthy man comes to the office due to worsening muscle pain and weakness over the past 6 months.  Carrying heavy items such as groceries has become increasingly difficult, and any prolonged activity causes diffuse muscle cramps.  He has also had a 4.5-kg (10-lb) weight gain over this time.  The patient takes no medications and does not use tobacco, alcohol, or illicit drugs.  Blood pressure is 140/90 mm Hg and pulse is 56/min.  BMI is 26 kg/m².  Physical examination shows muscle weakness, predominantly of the shoulder and hip girdles.  When the knee reflex is elicited, the quadriceps muscle contracts briskly but relaxation takes longer than normal.  There are splitting and peeling of the nails.  Serum creatine kinase is elevated.  Which of the following tests is most likely to identify the cause of this patient's muscle weakness?

A) 24-hour urinary cortisol excretion
B) Acetylcholine receptor antibody titers
C) CT scan of the chest
D) Muscle biopsy
E) Serum TSH level

Correct Answer:

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