A 7-day-old premature infant born at 26 weeks of gestation now has a grossly bloody stool, abdominal distention, and increasing oxygen requirements. choose the best initial diagnostic step in the evaluation of the patient's apparent GI hemorrhage.

Necrotizing enterocolitis (NEC) is a life-threatening condition seen mostly in premature infants. Although the precise etiology is unknown, contributing factors include GI tract ischemia, impaired host immunity, the presence of bacterial or viral pathogens, and the presence of breast milk or formula in the gut. Findings include bloody stools, abdominal distension, hypoxia, acidosis, and emesis. The initial diagnostic test of choice is plain film radiographs. The characteristic finding in NEC is pneumatosis intestinalis; free air in the peritoneum may also be seen. Perforation is a surgical emergency, otherwise observation and antibiotics are indicated.