Mark, a 3-day-old male neonate, is brought to the hospital by his parents due to concerns that he has not passed meconium since birth. His abdomen has become progressively distended, and he has experienced two episodes of green, bilious vomiting in the last 24 hours. He was born full-term via vaginal delivery with normal Apgar scores. He has been feeding poorly and appears increasingly irritable. Medical and Family History: No significant medical history or genetic conditions. Parents are first-time caregivers with no family history of similar neonatal presentations. 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, tenderness on palpation, and no organomegaly. Rectal Examination: Tight anal sphincter with explosive release of stool and gas upon withdrawal of the finger. Diagnostic Findings: Biopsy: Absence of ganglion cells in the submucosal and myenteric plexuses, with hypertrophied nerve fibers confirmed on histopathology. Question: What is the most common site of involvement in this disorder?
