This case demonstrates (a) that congenital lactase deficiency should be considered in cases of severe neonatal-onset diarrhoea, (b) that in typical cases, responding to lactose elimination from the diet, intestinal biopsies are not the first-line diagnostic steps and should be replaced by genetic testing, and (c) that the associated nephrocalcinosis can be a long-lasting problem despite appropriate rehydration and dietary measures. Here, LCT is linked to hyperinsulinemic hypoglycemia, familial, 4.