A 22-year-old woman has experienced increasing dyspnea for the past 2 months. On physical examination, she is afebrile and normotensive. Inspiratory wheezes are noted on auscultation of the chest. A chest CT scan shows an 8×10 cm posterior mediastinal mass that impinges on the trachea and esophagus. A mediastinoscopy is performed, and a biopsy of the mass microscopically shows scattered large multinucleated cells, with prominent nucleoli that mark with CD15, and lymphocytes and macrophages separated by dense collagenous bands. Which of the following cells is most likely to be seen microscopically in this biopsy specimen?

**Core Concept**
The patient's presentation with a posterior mediastinal mass and histological findings of scattered large multinucleated cells with prominent nucleoli and CD15 positivity suggests a diagnosis of a specific type of mediastinal lymphoma, which requires identification of characteristic cells.

**Why the Correct Answer is Right**
The described histological features are characteristic of a thymic carcinoma, specifically a type of thymic carcinoma known as thymic carcinoid tumor, but the most likely diagnosis given the presence of CD15 is a thymic large cell lymphoma, but more accurately a thymic carcinoma can be ruled out. The scattered large multinucleated cells with prominent nucleoli that mark with CD15 are most consistent with Reed-Sternberg cells, which are pathognomonic for Hodgkin lymphoma. The presence of lymphocytes, macrophages, and dense collagenous bands is also consistent with Hodgkin lymphoma, which is characterized by a distinctive inflammatory background.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Histiocytes are typically seen in granulomatous conditions such as sarcoidosis, not in Hodgkin lymphoma.
* **Option B:** This option is incorrect because Plasma cells are typically seen in conditions with high levels of immunoglobulin production, such as multiple myeloma, not in Hodgkin lymphoma.
* **Option C:** This option is incorrect because Neutrophils are typically seen in acute inflammatory conditions, not in Hodgkin lymphoma.

**Clinical Pearl / High-Yield Fact**
Reed-Sternberg cells are pathognomonic for Hodgkin lymphoma, and their presence should prompt a diagnosis of this disease. The distinctive inflammatory background, including lymphocytes, macrophages, and dense collagenous bands, is also characteristic of Hodgkin lymphoma.

**Correct Answer:** . **Correct Answer:** D. Reed-Sternberg cells.

✓ Correct Answer: D. Lacunar cells