A 3-year-old boy is brought to the pediatrician because his mother noticed an abnormal mass in his scrotum while changing his diapers. Fuher workup demonstrates elevated levels of serum a-fetoprotein. Which of the following is the most likely diagnosis?

Endodermal sinus (yolk sac) tumor is the most common malignant germ cell tumor of the testis in infancy and early childhood. It is characterized by an increase in serum a-fetoprotein, as well as histologically stainable a1 -antitrypsin. ENDODERMAL SINUS TUMOR Choriocarcinoma is less common in this age group and often results in an increase in hCG. Although hepatocellular carcinomas also elaborate a-fetoprotein, this is very unlikely in this patient's clinical setting. Leydig cell tumors are derived from testicular stroma and produce androgens and estrogens, often presenting with precocious pubey. Teratomas contain multiple germ layers and numerous tissues, including hair, teeth and sebaceous tissue. Unlike the teratomas encountered in women, most teratomas in men are malignant.