A 29-year-old radio disc jockey comes to your office because…

A 29-year-old radio disc jockey comes to your office because he has noticed a mass on his testicle. He denies fever or chills. He is in a monogamous relationship with his wife. They have been married for 5 years and have been unable to get pregnant, although they have been trying for the past 2 years. On physical examination, the appearance of his penis is normal. He has a soft rubbery mass in the left scrotum, near the testis. It does not transilluminate. When the patient is supine, the mass slowly collapses. What is your most likely diagnosis?