Behind the Swelling: Primary Mediastinal Large B-cell Lymphoma Masquerading as an Infection.

We present the case of a 26-year-old female patient with a history of asthma who initially presented with symptoms suggestive of a common infection. Her constellation of facial swelling, productive cough, and fatigue, coupled with imaging findings, first led to a diagnosis of right upper lobe pneumonia and suspected cellulitis. Despite appropriate antibiotic therapy, her symptoms persisted, and her clinical picture became more complex with the identification of Actinomyces in her sputum, shifting the differential diagnosis toward a more indolent cervicofacial infection. However, a progressively worsening and profound leukocytosis, along with a lack of clinical improvement, prompted further investigation. This ultimately revealed a large mediastinal mass, and subsequent tissue analysis confirmed a diagnosis of primary mediastinal large B-cell lymphoma. This case highlights a critical diagnostic challenge, illustrating how a rare and aggressive malignancy can masquerade as a common infectious process in a young, otherwise healthy individual, leading to a significant delay in diagnosis and treatment.