Epididymitis: This is the most common cause of acute testicular pain and swelling in young adults. It is usually caused by a bacterial infection, often related to a urinary tract infection or a sexually transmitted infection. The patient's age and symptoms align with this diagnosis.

Testicular Torsion: Although less common, testicular torsion is a significant consideration due to its urgency. It occurs when the spermatic cord becomes twisted, cutting off the blood supply to the testicle. Prompt diagnosis and treatment are crucial to save the affected testicle.
Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections. It can present with pain and swelling, similar to epididymitis, and may occur alongside epididymitis (epididymo-orchitis).
Hydrocele: An accumulation of fluid in the tunica vaginalis, a potential space around the testicle. While it can cause swelling, it typically does not cause acute pain unless it becomes large enough to exert pressure on the testicle or surrounding structures.

Testicular Tumor: Although less common, testicular cancer can present with pain and swelling. It is crucial to perform a thorough examination and possibly imaging studies to rule out a tumor, especially if the swelling does not resolve with treatment for other conditions.
Appendicitis (with a retrocecal appendix): In rare cases, an inflamed appendix located behind the cecum can irritate the testicle or surrounding structures, causing referred pain to the testicle. This diagnosis is less likely but should be considered, especially if abdominal symptoms are present.

Henoch-Schönlein Purpura (HSP): A form of vasculitis that can affect various parts of the body, including the testicles, leading to pain and swelling. It is more common in children but can occur in adults.
Spermatocele: A cystic structure filled with sperm that arises from the head of the epididymis. While it can cause swelling, it typically does not cause acute pain unless it becomes large or inflamed.

For the single most likely diagnosis, Epididymitis, the initial treatment often involves antibiotics, such as:

Doxycycline for sexually transmitted infections (STIs) or
Ciprofloxacin or Levofloxacin for infections not related to STIs, alongside pain management with NSAIDs (Non-Steroidal Anti-Inflammatory Drugs) like Ibuprofen. However, the choice of antibiotic should be guided by local resistance patterns and the suspected cause of the infection. It's also crucial to consider the patient's sexual history and possibly treat for STIs if appropriate. Always consult local guidelines and consider consulting a specialist if the diagnosis is uncertain or if the patient does not respond to initial treatment.