De Quervain's Tenosynovitis: This condition, which affects the tendons on the thumb side of the wrist, is a common cause of lateral forearm pain and swelling due to inflammation of the tendon sheath. It is often associated with repetitive motion or direct injury to the area.

Scaphoid Fracture: A fracture of the scaphoid bone, one of the carpal bones in the wrist, can cause pain and swelling on the lateral side of the forearm. This injury is common in individuals who fall onto an outstretched hand.
Wrist Sprain: A sprain of the wrist, which involves the stretching or tearing of ligaments, can lead to lateral forearm pain and swelling, depending on the specific ligaments involved.
Ganglion Cyst: A ganglion cyst is a lump or mass that forms under the skin, often on the wrist or finger, which can cause pain and swelling if it presses on nearby nerves or tendons.

Compartment Syndrome: Although less common, compartment syndrome is a serious condition that occurs when pressure within the muscle compartments increases, potentially cutting off blood flow. It requires immediate medical attention to prevent permanent damage.
Infection (Cellulitis or Abscess): An infection in the forearm can cause significant pain, swelling, and redness. Early diagnosis and treatment are crucial to prevent the spread of infection.
Osteomyelitis: Bone infection can occur in the forearm bones and may present with pain, swelling, and systemic symptoms like fever. It is a serious condition that requires prompt antibiotic treatment.

Eosinophilic Fasciitis: A rare condition characterized by inflammation of the fascia, the tissue surrounding muscles, which can cause pain, swelling, and skin changes in the affected area.
Tumor (Benign or Malignant): Although rare, tumors of the bone, soft tissue, or nerve can cause localized pain and swelling. A thorough diagnostic workup is necessary to rule out such conditions.