Gout: This condition is a common cause of acute joint swelling and pain, especially in the absence of trauma. The proximal phalanx is not a typical location, but gout can affect any joint. A young male with no past medical history could be experiencing his first gout attack.

Septic Arthritis: Although less common, septic arthritis is a possible cause of joint swelling and should be considered, especially if there's a history of recent infection or IV drug use.
Rheumatoid Arthritis (RA): RA can cause symmetric joint swelling, but it can also start with a single joint. The proximal phalanx is not the most common location, but RA should be considered in the differential.
Psoriatic Arthritis: This condition can cause joint swelling and is often associated with psoriasis. However, it can occur without skin symptoms.

Osteomyelitis: This is a bone infection that can cause swelling and pain. It's less common but can be serious if not treated promptly.
Bone Tumor: Although rare, a bone tumor (e.g., osteosarcoma) could cause swelling of the proximal phalanx. It's essential to consider this possibility to avoid delayed diagnosis.
Infectious Tenosynovitis: This condition involves infection of the tendon sheath and can cause swelling and pain.

Sarcoidosis: This condition can cause joint swelling, but it's relatively rare and often associated with other systemic symptoms.
Hemochromatosis: This genetic disorder can cause joint pain and swelling due to iron accumulation, but it's rare and usually associated with other symptoms.
Sjögren's Syndrome: This autoimmune disorder can cause joint pain and swelling, but it's relatively rare and often associated with dry eyes and mouth.