Review Question - QID 109239

A 4-day-old girl presents with her parents to the pediatrician for a routine visit. She was feeding well until this morning when she had several episodes of vomiting. The parents describe the vomitus as “bright green” and the patient has shown little interest in feeding since then. She was born at 36 weeks gestation to a 37-year-old G2P2 mother. The pregnancy was complicated by gestational diabetes, and all prenatal testing was unremarkable. The patient has not yet regained her birth weight and is in the 46th percentile for height and the 36th percentile for weight. The patient’s temperature is 99.2°F (37.3°C), blood pressure is 68/46 mmHg, pulse is 132/min, and respirations are 32/min. On physical exam, the patient is in mild distress. She has no dysmorphic features. Her abdomen is distended and firm without guarding. Bowel sounds are hypoactive. The patient’s abdominal radiograph can be seen in Figure A and the patient’s upper gastrointestinal series can be seen in Figure B. Which of the following is the most likely etiology of this patient’s condition?