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 18-year-old woman comes to her family physician after experiencing severe leg cramps when running to catch a bus. She recounts a similar recent episode that also included a burning sensation in her legs after running with a friend for half a mile. The next day her legs felt weak and sore. The patient is able to take long, slow walks without experiencing cramping or burning. A muscle biopsy is obtained. Laboratory studies reveal:
Hemoglobin: 13.8 g/dL
Creatine kinase: 4279 U/L
A urinalysis reveals myoglobinuria (myoglobin 761 ng/mL).
The enzyme that is most likely deficient in this disease is found in which part of the cell?