Mixed phenotype acute leukemia mimicking adult-onset Still's disease in a pregnant female: A rare case report.

INTRODUCTION: Mixed phenotype acute leukemia (MPAL) is an uncommon hematologic malignancy characterized by the coexistence of lymphoid and myeloid lineage markers. Although adult-onset Still's disease (AOSD) is a diagnosis of exclusion, MPAL may rarely present with similar laboratory and clinical findings, resulting in delayed diagnosis and treatment. CASE PRESENTATION: We report a case of a 36-year-old woman who presented at 20 weeks of gestation with inflammatory polyarthritis, hyperferritinemia, lymphadenopathy, thrombocytopenia, and leukocytosis. The patient fulfilled the Yamaguchi criteria for AOSD after exclusion of autoimmune and infectious conditions and was initially treated with hydroxychloroquine and corticosteroids. An inefficient response to treatment and persistent cytopenias led to reevaluation. A repeat peripheral smear revealed blast cells, and flow cytometry confirmed MPAL. Gestation-compatible chemotherapy protocols resulted in a favorable clinical and hematologic response, with no adverse maternal or fetal outcomes. CLINICAL DISCUSSION: The clinical overlap between the manifestations of AOSD and MPAL may occasionally result in misdiagnosis and delayed detection of an underlying malignancy. In patients presenting with presumed AOSD, careful exclusion of hematologic cancers is essential, particularly in the presence of atypical features such as persistent systemic inflammation, cytopenias, or poor therapeutic response. CONCLUSION: This case describes a rare association between the manifestations of AOSD and MPAL. In addition, it highlights the importance of recognizing warning signs that may indicate an underlying malignancy in patients presenting with AOSD-like symptoms.