Medial ankle sprain: This is often the most common cause of medial malleolus pain, especially in individuals who have recently experienced a traumatic event or injury to the ankle. The medial ankle sprain involves the deltoid ligament, which can cause pain and swelling on the medial side of the ankle.

Stress fracture of the medial malleolus: Overuse or repetitive stress on the bone can lead to a stress fracture, which is a common cause of medial malleolus pain in athletes or individuals who have recently increased their physical activity.
Tendinitis of the posterior tibial tendon: The posterior tibial tendon runs behind the medial malleolus and can cause pain in this area if it becomes inflamed or irritated.
Osteoarthritis of the ankle: Wear and tear on the ankle joint can cause pain and stiffness, especially on the medial side, where the deltoid ligament and surrounding bones are located.

Osteomyelitis or septic arthritis: Although less common, infections such as osteomyelitis (bone infection) or septic arthritis (joint infection) can cause severe pain and swelling in the medial malleolus area and require prompt medical attention to prevent serious complications.
Deep vein thrombosis (DVT): DVT can cause pain and swelling in the leg, including the area around the medial malleolus, and is a medical emergency due to the risk of pulmonary embolism.
Compartment syndrome: This is a condition where increased pressure within the muscle compartments of the leg can cause severe pain, swelling, and potentially permanent damage if not treated promptly.

Tumor (benign or malignant): Although rare, tumors can occur in the bones or soft tissues around the medial malleolus, causing pain and swelling.
Gout or pseudogout: These are forms of arthritis that can cause sudden, severe pain and swelling in the joints, including the ankle.
Nerve entrapment or neuropathy: Conditions such as tarsal tunnel syndrome can cause pain, numbness, or tingling in the foot and ankle, including the medial malleolus area.