Medial Meniscus Tear: The medial meniscus is a cartilage structure in the knee joint that can tear due to injury or degeneration, leading to pain on the interior side of the knee, swelling, and difficulty bearing weight. This diagnosis is likely due to the specific location of the pain and the symptoms described.

Medial Collateral Ligament (MCL) Sprain: An MCL sprain can cause pain on the inner aspect of the knee, swelling, and instability, making it difficult to bear weight. This injury often occurs due to a valgus stress (a force that bends the knee inward).
Osteoarthritis (OA) of the Knee: OA can lead to knee swelling and pain, particularly on the medial (inner) side, due to the uneven wear and tear of the joint cartilage. Weight-bearing difficulties can also be a symptom, especially in advanced stages.
Medial Tibial Stress Syndrome (Shin Splints): Although more commonly associated with pain in the lower leg, in some cases, medial tibial stress syndrome can refer pain to the knee area, causing swelling and discomfort on the medial side.

Septic Arthritis: An infection within the knee joint, which is a medical emergency. Symptoms can include sudden onset of knee pain, swelling, redness, and warmth, along with difficulty moving the knee. Fever may also be present.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg, which can cause swelling, pain, and warmth in the affected leg. Although DVT typically presents with calf pain, it can sometimes refer pain to the knee area.
Osteonecrosis (Avascular Necrosis) of the Knee: A condition where the bone tissue dies due to lack of blood supply, leading to knee pain and swelling. It can be a complication of steroid use, excessive alcohol consumption, or severe trauma.

Tumor (Benign or Malignant): Although rare, tumors can cause knee pain and swelling. The symptoms might be similar to those of more common conditions but could also include night pain, unexplained weight loss, or a palpable mass.
Hemarthrosis: Bleeding into the joint space, which can be due to trauma, bleeding disorders, or anticoagulant therapy. It leads to sudden swelling and pain in the knee.
Pigmented Villonodular Synovitis (PVNS): A rare condition characterized by the overgrowth of the joint lining (synovium), leading to knee swelling, pain, and potentially joint instability.