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 <i>Actinomyces</i> 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.