Muscular dystrophies and mitochondrial myopathies Quiz: Ace Your Exams

A 3-year-old boy is brought to the office by his parents due to difficulty keeping up with other children at daycare. Over the last 2 months, the daycare teachers have noticed that the patient cannot walk independently and seems more tired than usual. The parents have noticed that the boy finds it difficult to climb stairs. He has been eating and growing normally, and he has no past medical history except for an upper respiratory tract infection 3 weeks ago that resolved on its own. Family history is noncontributory. Temperature is 37°C (98.6°F), pulse is 70/min, respirations are 19/min and blood pressure is 100/60 mmHg. Physical examination shows enlarged and nontender calves bilaterally. When asked to rise from the floor, the boy uses hand support to push himself to an upright position. The remainder of the examination is unremarkable. Serum creatinine kinase is 14,200 U/L. Which of the following is the most likely diagnosis in this patient?