Peroneal Tendinopathy or Tendonitis: This condition is the most likely diagnosis due to the location of the symptoms. The peroneal tendons run posterior to the lateral malleolus and are retained by the inferior peroneal retinaculum. Inflammation or degeneration of these tendons can cause swelling and tenderness in this area, often due to overuse or repetitive ankle motion.

Peroneal Tenosynovitis: This is an inflammation of the tendon sheath surrounding the peroneal tendons. It can cause similar symptoms to peroneal tendinopathy and is often related to repetitive activities or trauma.
Ankle Sprain: Although more commonly associated with lateral ankle pain, a severe ankle sprain can cause swelling and tenderness that might involve the area around the inferior peroneal retinaculum, especially if there's associated peroneal tendon injury.
Peroneal Subluxation or Dislocation: This condition involves the peroneal tendons slipping out of their normal position behind the lateral malleolus, which can cause acute pain and swelling in the area of the inferior peroneal retinaculum.

Osteochondral Lesion of the Talus or Fibula: Although less common, an osteochondral lesion (a piece of cartilage and bone that has been damaged) in the ankle can cause deep ankle pain and might refer pain to the area around the inferior peroneal retinaculum. Missing this diagnosis could lead to prolonged pain and potential for further joint damage.
Infection (Cellulitis or Abscess): Infection in the soft tissues around the ankle can cause swelling, redness, and tenderness. It's crucial to identify and treat infections promptly to prevent serious complications.
Tumor: Although rare, a soft tissue tumor (benign or malignant) in the ankle region could cause localized swelling and pain. Early detection is critical for effective management.

Rheumatologic Conditions (e.g., Rheumatoid Arthritis, Psoriatic Arthritis): These conditions can cause widespread joint pain and swelling, including the ankle. However, they would typically present with symptoms in multiple joints rather than being isolated to the area around the inferior peroneal retinaculum.
Nerve Entrapment (e.g., Superficial Peroneal Nerve Entrapment): This condition involves compression of a nerve and can cause pain, numbness, and tingling in the area supplied by the nerve. It's less common in the context of swelling and tenderness specifically around the inferior peroneal retinaculum.