Background: Congenital lactase deficiency (CLD) is a rare disorder that is characterized by severe osmotic diarrhea and malnutrition on the first day of birth. Clinical findings occur due to the defective digestion of the main carbohydrate, lactose, in breast milk. This autosomal recessive disorder is caused by variants in lactase-phlorizin hydrolase (LCT).
Case Report: We report the first genetically confirmed case of CLD in Turkey. The patient suffered from watery diarrhea after breastfeeding, which ceased after feeding the formula containing lactose-free hydrolyzed cow’s milk. The lactose chal-lenge test demonstrated a lactose intolerance pattern. A novel homozygous variant was detected in LCT.
Conclusion: Although genetic analyses are important to highlight underlying etiologies of congenital diarrhea, it should be remembered that clinical findings of patients, fecal characteristics, and the effects of dietary treatment are the primary and most important steps that lead to an accurate diagnosis.