Rash, Fever, and a Diagnostic Puzzle: Recognizing MIS-C Among Mimicking Illnesses in a Child After COVID-19 Exposure — A Case Report

Abstract

Multisystem inflammatory syndrome in children (MIS-C) is a delayed hyperinflammatory condition following SARS-CoV-2 infection that can mimic Kawasaki disease, sepsis, and toxic shock. A previously healthy 9-year-old boy presented on illness day 5 with fever (102.9°F), non-pruritic maculopapular rash, bilateral ankle pain with refusal to walk, and exertional dyspnea. Initial findings included room-air oxygen saturation of 87% with tachypnea (respiratory rate in the 40s), requiring supplemental oxygen via oxymask (2-3 L/min); ICU-level care was not required. Laboratory evaluation revealed markedly elevated C-reactive protein (25.16 mg/dL; reference ≤0.90), erythrocyte sedimentation rate (50 mm/hr; reference 0-15), transaminitis, direct hyperbilirubinemia, positive rapid streptococcal antigen, and anti-streptolysin O titer 1,650 IU/mL (reference <200). Chest radiography showed hypoinflated lungs with prominent vasculature but no consolidation. Echocardiography revealed elevated right ventricular systolic pressure (approximately half to two-thirds systemic) with preserved biventricular function and no coronary artery abnormalities. Despite azithromycin, ceftriaxone, and piperacillin-tazobactam, inflammatory markers and hepatic function worsened. On admission day 4, rheumatology elicited a history of COVID-19 exposure approximately 2 weeks earlier. Additional testing showed elevated D-dimer (6,048 ng/mL FEU; reference 215-499), ferritin (471.8 ng/mL; reference 16-300), lactate dehydrogenase (480 U/L; reference 110-260), and B-type natriuretic peptide (156 pg/mL). Intravenous methylprednisolone 1 mg/kg every 12 hours led to rapid defervescence, respiratory improvement, and laboratory normalization. Anti-nucleocapsid SARS-CoV-2 antibodies later returned positive, consistent with prior natural infection. This case highlights how concurrent streptococcal carriage and hepatic dysfunction may obscure MIS-C and underscores the importance of exposure history, inflammatory testing, and early corticosteroid therapy.