Review Question - QID 212382

A 6-month-old boy presents to his pediatrician for a wellness examination. The mother reports that her child has difficulty rolling from his back to his front and sitting unsupported. The patient is able to smile and furrow his brow normally, but she has noticed that he has a weak cry and suck. He was born at 38 weeks gestation via a spontaneous vaginal delivery without any complications. The mother said that the patient appeared "normal" until the past few weeks. On physical exam, his extraocular muscle movements are intact, and a symmetric smile is seen. He has symmetric flaccid weakness of both his upper and lower extremities. He also has a bell-shaped chest. Deep tendon reflexes are diminished. Which of the following is the most likely cause of this patient's symptoms?