Concurrent serum sickness–like reactions in fraternal twins following amoxicillin exposure

Serum sickness–like reaction (SSLR) is an immunologic process most often triggered by β-lactam antibiotics or viral infections, typically presenting with urticarial rash, arthralgias, and low-grade fever. This report describes concurrent SSLR in 5-year-old fraternal twin girls after amoxicillin exposure for presumed viral pharyngitis. Twin A developed diffuse urticaria, facial and extremity swelling, arthralgias, and mucosal discomfort. Laboratory studies showed leukocytosis, elevated inflammatory markers, and detection of rhinovirus/enterovirus by PCR. She required hospitalization, systemic corticosteroids, and multimodal analgesia, with gradual improvement over several days. Twin B developed similar symptoms three days later, including rash, edema, and joint pain, but had a milder course managed with antihistamines and supportive care. The simultaneous occurrence of SSLR in siblings is rarely reported, and concurrent presentation in fraternal twins has not been described in the literature. This case highlights the interplay of shared environmental exposures, potential infectious triggers, and host susceptibility in SSLR, as well as the variability of disease expression even among related children. Awareness of SSLR in the differential diagnosis of rash and arthralgia following antibiotic exposure can prevent unnecessary investigations, guide supportive treatment, and reduce morbidity.