Neuromuscular Disorders USMLE Step 1 Practice Question

A 6-year-old boy is brought to the emergency department by his parents after he fell while playing soccer and was unable to get up. He has a 10-month history of progressive difficulty climbing stairs and rising from the floor. His parents report that when standing from a seated position on the ground, he "walks his hands up his legs" to push himself upright. On examination, he has a waddling gait, symmetric hip weakness (4/5 bilaterally), and markedly enlarged, firm calf muscles. Serum creatine kinase is 8,500 U/L. The patient's maternal uncle required a wheelchair by age 12 and died at age 18 from respiratory complications. Which of the following proteins is most likely deficient in this patient?