unmasking idiopathic secondary hemophagocytic lymphohistiocytosis in a young female: a diagnostic challenge presenting as pyrexia of unknown origin

Hemophagocytic lymphohistiocytosis (hlh) is a hyperinflammatory syndrome that can present as prolonged fever of unknown origin (fuo). we describe the case of a 26-year-old woman who presented with two months of intermittent high-grade fever, joint pain, and transient salmon-colored skin rashes. laboratory investigations revealed cytopenia, elevated triglycerides, abnormal liver enzymes, and a markedly elevated serum ferritin level (>10,000 ng/ml). extensive evaluation for infectious and autoimmune causes was negative. whole-body pet-ct demonstrated diffusely increased marrow activity with small mesenteric lymph nodes, while bone marrow examination confirmed hemophagocytosis. the patient met six of the hlh-2004 diagnostic criteria, and her hscore was calculated at 228, indicating a high probability of hlh. in the absence of any identifiable trigger, a diagnosis of idiopathic secondary hlh was made. she was treated with dexamethasone monotherapy, which led to rapid resolution of fever, normalization of laboratory parameters, and sustainedremission at three-month follow-up. this case underscores the importance of considering hlh early in adults with unexplained fever, cytopenia, and extreme hyperferritinemia, and highlights that corticosteroid monotherapy may be sufficient in selected idiopathic cases.