Complement C9 deficiency as a novel risk factor for invasive Candida esophagitis in children: a single case in-depth analysis

BACKGROUND: The complement system constitutes a critical component of innate immunity against fungal pathogens. Complement component 9 (C9), an essential constituent of the membrane attack complex (MAC), plays a pivotal role in pathogen elimination. However, isolated C9 deficiency has not been previously reported in association with fungal esophagitis. CASE PRESENTATION: We present the case of a nearly 7-year-old girl who suffered from progressive dysphagia, nausea, and vomiting for more than two months. Esophagogastroduodenoscopy revealed esophageal stricture, and histopathological examination of mucosal biopsies confirmed Candida albicans infection. Despite multiple endoscopic dilations, symptomatic recurrence occurred monthly. Whole-exome sequencing identified a pathogenic variant confirming isolated C9 deficiency. Combined systemic antifungal therapy with fluconazole and repeated endoscopic dilatation achieved sustained symptomatic resolution. CONCLUSION: Isolated C9 deficiency may represent a susceptibility factor for recurrent fungal esophagitis with stricture formation in children. Complement screening should be considered in unexplained recurrent cases. Combined antifungal therapy and endoscopic intervention achieved sustained remission in this patient.