A 7-day-old male infant, previously well with a history of mild transient tachypnea at birth, is brought to the emergency department after 12 hours of increasing bilious emesis, decreased urine output, and significant lethargy. His parents report worsening irritability and poor feeding. On examination, his temperature is 38.5°C, blood pressure is 70/40 mm Hg, pulse is 175/min, respiratory rate is 70/min, and oxygen saturation is 92% on room air. He appears mottled with poor peripheral perfusion and has a capillary refill time of greater than 4 seconds. His abdomen is markedly distended and tense, with diffuse tenderness, absent bowel sounds, and purpuric discoloration of the abdominal wall. Rectal examination reveals frank blood. An abdominal X-ray shows dilated bowel loops, clear evidence of pneumatosis intestinalis, and portal venous gas, but no free air. Given these critical findings, what is the most probable diagnosis and the immediate critical management strategy?

A 3-week-old male infant, born at 26 weeks gestation, is being evaluated on the pediatric ward for worsening feeding intolerance, characterized by increased gastric residuals and emesis after recent advancement of enteral feeds. He was born weighing 800 g (1 lb 12 oz) and now weighs 1,400 g (3 lb 1 oz). His birth was complicated by chorioamnionitis and he required initial mechanical ventilation, eventually weaned to a nasal cannula. For the past 12 hours, he has exhibited increased lethargy and temperature instability. His current vital signs are: temperature 36.0°C (96.8°F), blood pressure 68/40 mmHg, pulse 145/min, respirations 30/min, and SpO2 94% on 3L nasal cannula. On physical examination, the infant appears listless and pale. His abdomen is visibly distended, firm to palpation, and bowel sounds are diminished. A rectal examination reveals maroon-colored stool. Given these findings, what is the most likely diagnosis and the critical initial management step?