You are answering a USMLE-style medical question. Question: A 3-day-old male is evaluated in the hospital nursery for delayed passage of meconium. His mother is breastfeeding and has started to produce milk. The patient has been feeding well every two hours and is urinating over eight times per day. The patient was born at 35 weeks gestation to a 27-year-old gravida 4. The patient is of Ashkenazi Jewish descent, and the patient’s parents denied all prenatal genetic testing. The pregnancy was uncomplicated until the patient’s mother had spontaneous rupture of membranes at 35 weeks gestation. The patient’s three older siblings are all healthy. In the hospital nursery, the patient’s temperature is 98.2°F (36.8°C), blood pressure is 48/32 mmHg, pulse is 164/min, and respirations are 48/min. On physical exam, he appears to be in moderate distress. He has no dysmorphic features, and his abdomen is distended and non-tender. Bowel sounds are absent. Which of the following is the best next step in management? Options: A) Abdominal radiograph B) Contrast enema C) Upper gastrointestinal series D) Sweat testing Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.