Review Question - QID 210999

A 50-year-old man arrives to the clinic with progressive weakness. For 3 months, he has had difficulty climbing the stairs, which has now progressed to difficulty getting out of a chair. He denies diplopia, dysphagia, dyspnea, muscle aches, or joint pains. He denies weight loss, weight gain, change in appetite, or heat or cold intolerance. He reports intermittent low-grade fevers. He has a medical history significant for hypertension and hyperlipidemia. He has taken simvastatin and losartan daily for the past 6 years. His temperature is 99.0°F (37.2°C), blood pressure is 135/82 mmHg, and pulse is 76/min, respirations are 12/min, and oxygen saturation is 98% on room air. Cardiopulmonary examination is normal. The abdomen is soft, non-tender, non-distended, and without hepatosplenomegaly. Muscle strength is 3/5 in the hip flexors and 4/5 in the deltoids, biceps, triceps, patellar, and Achilles tendon reflexes are 2+ and symmetric. Sensation to pain, light touch, and vibration are intact. Gait is cautious, but grossly normal. There is mild muscle tenderness of his thighs and upper extremities. There is no joint swelling or erythema and no skin rashes. A complete metabolic panel is within normal limits. Additional lab work is shown below:

Serum:
Na+: 141 mEq/L
Cl-: 100 mEq/L
K+: 4.3 mEq/L
HCO3-: 23 mEq/L
Urea nitrogen: 18 mg/dL
Glucose: 128 mg/dL
Creatinine: 1.0 mg/dL
Alkaline phosphatase: 69 U/L
Aspartate aminotransferase (AST): 302 U/L
Alanine aminotransferase (ALT): 210 U/L
TSH: 6.9 uU/mL
Thyroxine (T4): 5.8 ug/dL
Creatine kinase: 4300 U/L
C-reactive protein: 11.9 mg/L
Erythrocyte sedimentation rate: 37 mm/h

Which of the following is the most accurate diagnostic test?