In congenital absence of the vas deferens, mutation of the cystic fibrosis transmembrane receptor gene (CFTg) occurs. The epididymis vas deferens, seminal vesicle, membranous urethra, part of the trigone of the bladder, and ureter arise from the mesonephric duct. In the presence of a normal FSH level, testicular biopsy would most likely confirm normal sperm formation. In the presence of a Sertoli cell tumor, spermatozoa are unlikely to form, and the FSH level is elevated.