Pain on the medial side of the knee in the right leg can be caused by the following: 1. Sprain of the medial collateral ligament. When there is a sprain of the medial collateral ligament, there can be pain in the medial ligament alignment of the knee joint, especially when the knee joint is turned outward, the pain is very significant. At this time, early examination and MRI of the knee joint is required for diagnosis and early treatment. 2. Goose foot sprain or goose foot inflammation, the goose foot is located on the medial side of the knee joint and ends at the proximal end of the tibia. If there is a sprain of the goose foot or a chronic inflammation of the goose foot, there can be pain at the medial goose foot stop of the knee joint. Injury to the medial meniscus. When the medial meniscus of the knee is injured or torn, there can be pain in the medial aspect of the knee, often when the knee is flexed or externally rotated, which is diagnosed and treated by early physical examination and early MRI of the meniscus.