Answer: B. Melanotic neuroectodermal tumor of infancy

Sections show a biphasic neoplasm involving the epididymis composed of nests of small round blue primitive cells with interspersed cords and tubules of epithelioid cells containing melanin pigment. Adenomatoid tumor is the most common paratesticular tumor in adults, but it is mesothelial in nature and lacks small round blue cell morphology and lacks pigment. Metastatic neuroblastoma is a consideration but the absence of a primary lesion elsewhere as well as negativity for PHOX2b argue against this. Embryonal rhabdomyosarcoma is the most common malignant paratesticular tumor in pediatric patients, but desmin and myogenin reactivity would be expected. This tumor shows classic morphology of melanotic neuroectodermal tumor of infancy, which most commonly occurs in the head and neck, but has been reported in the paratesticular region. The primitive blue cell component shows synaptophysin labeling and the epithelioid, pigmented component expresses cytokeratin and HMB-45. Metastatic melanoma is vanishingly rare in this age group, particularly without a known primary, and would be expected to show S100 positivity while typically lacking keratin expression.