testicular yolk sac tumor (MONDO_0003402) is a non-seminomatous malignant germ cell tumor arising from the testis. It affects infants, young children, and postpubertal males. It is the most frequently seen testicular neoplasm during childhood. The vast majority of patients present with an asymptomatic scrotal mass. The tumor mimics the yolk sac of the embryo and produces alpha-fetoprotein (AFP). It metastasizes to distant anatomic sites. Prognostic factors relate to the clinical stage and the degree of AFP elevation. Also known as: testicular yolk Sac neoplasm, testis yolk sac tumor, testis yolk sac tumour.