Epididymitis: This is the most likely diagnosis, as it is a common cause of scrotal pain in men, especially if an STI is suspected. Epididymitis is an inflammation of the epididymis, which can be caused by bacterial infections, including those transmitted sexually.

Orchitis: Inflammation of one or both testicles, which can be caused by viral or bacterial infections, including STIs. It often presents with scrotal pain and can occur with or without discharge.
Urethritis: Inflammation of the urethra, often caused by STIs, which can lead to symptoms like scrotal pain, dysuria, and sometimes discharge.
Prostatitis: Inflammation of the prostate gland, which can be caused by bacterial infection, including those transmitted sexually. It may present with scrotal pain, pelvic pain, and urinary symptoms.

Testicular Torsion: A medical emergency where the spermatic cord becomes twisted, cutting off the blood supply to the testicle. It presents with sudden, severe scrotal pain and requires immediate surgical intervention to save the testicle.
Appendicitis (if pain radiates or is localized to the lower right abdomen): Although less common, appendicitis can sometimes present with referred pain to the scrotum, especially if the appendix is located near the pelvic area.

Henoch-Schönlein Purpura (HSP): A form of vasculitis that can cause scrotal pain and swelling, along with the characteristic purpura (rash) and joint pain.
Spermatocele or Epididymal Cyst: Benign cysts that can cause scrotal pain or discomfort, though they are typically painless and discovered incidentally.
Mumps Orchitis: A viral infection that can cause orchitis, leading to scrotal pain and swelling, usually in post-pubertal males who have not been vaccinated against mumps.