Unmasking MHC Class II Deficiency: A Novel Mutation in a Child with Pediatric ARDS due to Pneumocystis Jirovecii Pneumonia.

An 8-month-old boy, the first child of a fourth-degree consanguineous couple with an uneventful past medical history, presented with fever and respiratory distress. He was intubated and managed with high-frequency ventilation. Chest radiography revealed bilateral white-out lungs, and his oxygenation index was 31. Pneumocystis jirovecii was identified through polymerase chain reaction of respiratory secretions. The child was treated with cotrimoxazole and systemic steroids. Due to the severity of the infection caused by an atypical organism, an underlying immunodeficiency was suspected. Genetic analysis revealed a novel homozygous mutation in the RFXANK gene, consistent with major histocompatibility complex class II deficiency. This case represents a rare inborn error of immunity with survival following a severe infection.