Review Question - QID 109596

An 11-year-old boy is brought to the pediatrician by his mother for vomiting. The patient has been vomiting for the past week, and his symptoms have not been improving. His symptoms are worse in the morning and tend to improve throughout the day. The patient also complains of occasional headaches and had diarrhea several days ago. The patient eats a balanced diet and does not drink soda or juice. The patient's brothers both had diarrhea recently that resolved spontaneously. His temperature is 99.5°F (37.5°C), blood pressure is 80/45 mmHg, pulse is 90/min, respirations are 16/min, and oxygen saturation is 98% on room air. On physical exam, the patient appears to be in no acute distress. A cardiopulmonary exam reveals a minor flow murmur. A neurological exam reveals cranial nerves 2-12 as grossly intact with mild narrowing of the patient's visual fields. The patient's gait is stable, and he is able to jump up and down. Which of the following is the most likely direct cause of this patient's presentation?