Malabsorptive diarrhea is a newly discovered autosomal recessive disorder that presents as congenital malabsorptive diarrhea and an almost complete absence of intestinal endocrine cells. The patients present in the first few weeks of life with vomiting, diarrhea, and a severe hyperchloremic metabolic acidosis after the ingestion of standard cow’s milk–based formulas. It results from loss of function mutations in the NEUROG3 gene. The histology of the small intestine shows a normal architecture, but with a severe dysgenesis of the endocrine cells. Enterocytes, Paneth cells, and goblet cells appear normal and there is no intraepithelial or lamina propria infiltration of inflammatory cells. However, there is an almost complete absence of endocrine cells and the rare endocrine cells that are present appear morphologically abnormal. These features can be highlighted using chromogranin immunostains (564).