A 62-year-old man develops scaling and non-scaling patches, and plaques over his chest and back. They are itchy, but not painful. The rest of the examination is normal, except for lymphadenopathy. Examinations of the blood film and skin biopsy histology, both, reveal unusually large monocytoid cells. (See Figure below) Which of the following is the most likely diagnosis?

**Core Concept**
Myocardial infiltration by large monocytoid cells in a patient with scaling and non-scaling patches, plaques, and lymphadenopathy suggests a systemic disease with cutaneous and cardiac involvement.

**Why the Correct Answer is Right**
The presence of large monocytoid cells in the blood film and skin biopsy histology, along with the clinical presentation of scaling and non-scaling patches, plaques, and lymphadenopathy, is suggestive of Myocardial Infiltration by Sézary cells, a hallmark of Sézary Syndrome, a cutaneous T-cell lymphoma. Sézary cells are malignant T-cells that circulate in the blood and skin, leading to the characteristic skin lesions and systemic involvement.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not account for the presence of large monocytoid cells and the systemic involvement of the disease.
**Option B:** This option is incorrect because it is a benign condition that does not typically present with lymphadenopathy or systemic involvement.
**Option C:** This option is incorrect because it does not explain the presence of large monocytoid cells and the characteristic skin lesions.

**Clinical Pearl / High-Yield Fact**
Sézary Syndrome is a type of cutaneous T-cell lymphoma that is characterized by the presence of Sézary cells in the blood and skin, and typically presents with scaling and non-scaling patches, plaques, and lymphadenopathy.

**Correct Answer:** C. Sézary Syndrome

✓ Correct Answer: C. primary cutaneous T-cell lymphoma