A 26-year-old man has occasionally felt pain in the scrotum for the past 3 months. On physical examination, the right testis is more tender than the left but does not appear to be enlarged. An ultrasound scan shows a 1.5-cm mass within the right testis. A right orchiectomy is performed, and gross examination shows the mass to be hemorrhagic and soft. A retroperitoneal lymph node dissection is done. In sections of the lymph nodes, a neoplasm is found with extensive necrosis and hemorrhage. Microscopic examination shows that areas of viable tumors are composed of cuboidal cells intermingled with large eosinophilic syncytial cells containing multiple dark, pleomorphic nuclei. Immunohistochemical staining of syncytial cells is most likely to be positive for which of the following?
High-Yield Explanation
Choriocarcinoma is the most aggressive testicular carcinoma. It often metastasizes widely. The primitive syncytial cells mimic the syncytiotrophoblast of placental tissue and stain for human chorionic gonadotropin. a-Fetoprotein is a marker that is more likely to be found in mixed tumors with a yolk sac component. Carcinoembryonic antigen (CEA) is found in a variety of epithelial neoplasms, particularly adenocarcinomas. CD20 is a lymphoid marker for B cells. Testosterone is found in Leydig cells. Vimentin is more likely to be seen in sarcomas, which are rare in the testicular region.