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Orchitis: Picture on left demonstrates testicular enlargement caused by infection within the body of the testis. The inflammation has spread from the testis to the skin of the scrotum, with resulting edema causing fewer skin folds over the right testicle. No transillumination is seen (picture on right) as the inflamed testis does not allow the passage of light (as opposed to hydrocele shown above, which readily conducts light). This is not always the case, as sometimes orchitis will cause a "reactive hydrocele" to form, which will transilluminate.

Image Credit

Charlie Goldberg, M.D.