Medial Collateral Ligament (MCL) Sprain: This is the most likely diagnosis because the MCL is the primary stabilizer against valgus stress, and injury to this ligament is common in sports and activities that involve twisting or bending. The presentation of medial knee pain with valgus stress is highly suggestive of an MCL sprain.

Medial Meniscus Tear: The medial meniscus can be injured with valgus stress, especially if there is a component of rotation. Symptoms may include locking, catching, or giving way, in addition to pain.
Pes Anserine Bursitis: This condition involves inflammation of the bursa located on the medial aspect of the knee, where the tendons of the sartorius, gracilis, and semitendinosus muscles insert. It can cause medial knee pain, especially with activities that involve bending or twisting.

Fracture (e.g., Medial Tibial Plateau Fracture): Although less common, a fracture must be considered, especially if there was a significant trauma or if the patient has risk factors for osteoporosis. A fracture can have serious implications if not promptly diagnosed and treated.
Infection (e.g., Septic Arthritis): Infection within the knee joint is a medical emergency. While it may present with systemic symptoms like fever, localized signs such as warmth, redness, and swelling around the knee, along with pain, especially with movement, should prompt consideration of this diagnosis.

Osteonecrosis of the Medial Femoral Condyle: This is a rare condition where the bone tissue of the medial femoral condyle dies due to lack of blood supply. It can cause medial knee pain and should be considered in patients with risk factors such as long-term steroid use or excessive alcohol consumption.
Tumor (e.g., Osteosarcoma): Although extremely rare, a tumor should be considered in the differential diagnosis of knee pain, especially if the pain is persistent, worsening, or accompanied by other systemic symptoms like weight loss or night pain.