This week’s edition of the Step 1 Question of the Week is about biochemistry. See if you can answer the question and then watch the video to learn the correct answer as well as the rationales for each option.
An 11-year-old boy is brought to his pediatrician by his mother because of recent episodes of lightheadedness, racing pulse, dizziness, and nausea. On physical examination, there is generalized decreased muscle tone, and the liver and spleen are palpated a few centimeters below the left costal margin. Laboratory studies show increased creatinine kinase and decreased glucose levels. Urine ketones are positive, with negative glucose. An electromyogram shows features of myopathy.
Which of the following enzymes is most likely deficient in this patient?