A neonate presents with soft, non-tender scrotal swelling that transilluminates. What is the most likely diagnosis?

The presence of a soft, non-tender scrotal swelling in a neonate that transilluminates is most consistent with a hydrocele. A hydrocele occurs when fluid accumulates in the tunica vaginalis, the protective sheath surrounding the testis. The transillumination test is a key clinical finding; when a light is shone through the scrotum, the fluid in the hydrocele allows the light to pass through, confirming the nature of the swelling as fluid-filled rather than solid or complex.

In contrast, conditions such as cryptorchidism, hypospadias, and testicular torsion would present differently. Cryptorchidism is characterized by an absent testicle in the scrotum, and although it might cause swelling, it would not typically transilluminate because it involves solid tissue rather than fluid. Hypospadias refers to a congenital condition affecting the placement of the urethral opening and does not cause scrotal swelling that transilluminates. Testicular torsion usually presents with sudden, severe pain and the affected testicle may appear elevated and firm, also showing no transillumination effect.

Therefore, the characteristics of the swelling and the positive transillumination sign lead to the diagnosis of