What is the diagnosis? Follow us and come back next week for the answer! | Presented by Stephanie Shea MD, under the supervision of Fumiko Dekio MD, while on the Pediatrics rotation. | Age: 3 years | Sex: Female | Site: Posterior mediastinum | Procedure: Biopsy, followed by resection | Clinical history: Three day history of right-sided chest pain. History was negative for trauma, rash, fever, difficulty breathing, upper respiratory symptoms, or environmental exposures. Physical exam found mild decrease in air entry on the right hemithorax, especially in the lower zone. | Radiographic description: Posterior mediastinal mass extending into the spinal canal. A few punctate calcifications and foci of cystic degeneration are seen within the inferior aspect of the mass. | Gross description: A 9x6x3cm solitary mass. The majority of the parenchyma is dark red/brown in color, with dispersed focal areas of tan coloration/heterogeneity appreciated. A tan capsule overlies the anterior aspect only, and continues internally to form a multi-loculated appearance near the anterior surface. | NSE: positive | S100: positive in schwannian stroma and large tumor cells | CD56: positive | Chromogranin: positive | Synaptophysin: positive | LCA: highlights co-existing lymphocytes | CD99: negative | TdT: negative | AE1/3: negative | MYF4: negative.