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?