Medullary haemorrhagic interstitial nephritis in multisystem inflammatory syndrome in adults following COVID-19.

Multisystem inflammatory syndrome in adults (MIS-A) is a severe postinfectious complication of COVID-19. Renal involvement is common; however, histopathological findings have not been adequately characterised. We here report a case of a previously healthy man in his 60s who developed persistent fever, diarrhoea, glossitis and digital swelling approximately 3 weeks after COVID-19 infection. Antibiotic therapy was ineffective and renal function deteriorated. He was diagnosed with MIS-A and corticosteroid therapy produced marked clinical improvement. He was discharged 3 weeks after hospital admission. Renal biopsy demonstrated medullary haemorrhagic interstitial nephritis with preserved cortex, an uncommon pathological finding in renal injury secondary to COVID-19, which is consistent with possible immune-mediated endothelial injury.This case highlights medullary haemorrhagic interstitial nephritis as a potential pathological manifestation of MIS-A and illustrates a diagnostic pitfall. Recognition of MIS-A is essential for timely corticosteroid treatment and prevention of life-threatening multiorgan failure.