Clinical diagnosis
Labs to assess for consequences of GI losses (e.g. BMP, Mg/Phos, VBG) +/- workup to exclude alternative diagnoses
Food protein-induced enterocolitis syndrome
Esophageal stenosis/atresia
Neutropenic enterocolitis 
Pyloric stenosis
Necrotizing enterocolitis
Intestinal stenosis/atresia
HIV diarrhea
Duodenal atresia