Mark, a 3-day-old male neonate, is brought to the hospital by his parents because he has not passed meconium since birth. They report progressively worsening abdominal distension and two episodes of green, bilious vomiting in the past 24 hours. Mark was born full-term via vaginal delivery with no complications and normal Apgar scores. He has been feeding poorly and appears increasingly irritable. Medical and Family History: No significant medical or family history of similar conditions. Physical Examination: Vitals: Temperature: 98.6°F, HR: 130 bpm, RR: 50 breaths/min, BP: 70/40 mmHg. General Appearance: Irritable neonate with poor feeding and abdominal distension. Abdomen: Markedly distended with visible bowel loops, tympanic to percussion, and tenderness on palpation. No organomegaly. Rectal Examination: Tight anal sphincter with explosive release of stool and gas upon finger withdrawal. You suspect congenital aganglionic megacolon (Hirschsprung disease) in this patient. Which of the following is the definitive diagnostic test for Hirschsprung disease?
