Infectious Tenosynovitis: This condition, an infection of the tendon sheath, is a common cause of a warm, stiff, and swollen finger, especially in the absence of trauma. The symptoms can develop over a few days, and the infection can be caused by bacteria, often introduced through a small puncture wound.

Rheumatoid Arthritis (RA): Although less common in a single finger presentation, RA can cause stiffness, swelling, and warmth in the joints. It's an autoimmune disease that can start at any age, including in young adults.
Osteoarthritis (OA): Though OA is more common in older populations, it can occur in younger individuals, especially if there's a history of trauma or certain occupations. However, the absence of trauma in this case makes it less likely.
Gout: Gouty arthritis can cause sudden onset of joint pain, swelling, redness, and warmth, typically affecting the base of the big toe but can affect other joints, including fingers.
Psoriatic Arthritis: This condition combines the swollen, scaly skin of psoriasis with joint pain and arthritis symptoms. It could cause finger swelling and stiffness, especially if the patient has a history of psoriasis.

Septic Arthritis: Although less likely given the lack of systemic symptoms, septic arthritis is a medical emergency that requires prompt diagnosis and treatment to prevent joint destruction. It presents with sudden onset of pain, swelling, and decreased range of motion in a single joint.
Osteomyelitis: Bone infection can cause localized pain, swelling, and warmth. It's crucial to consider, especially if there's a history of recent infection or compromised immune status.
Deep Vein Thrombosis (DVT): While DVT typically affects the legs, it can occur in the arms. Swelling and warmth could be part of the presentation, although pain and discoloration are more common symptoms.

Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the joints, leading to swelling and stiffness. It's less common and would typically be associated with other systemic symptoms.
Sjögren's Syndrome: An autoimmune disorder that can cause arthritis among other symptoms. It's rare and usually presents with dry eyes and mouth.
Relapsing Polychondritis: A rare disease that causes recurring episodes of cartilage inflammation, which can affect various parts of the body, including joints, and cause swelling and stiffness.