Pes anserine bursitis: This is the most likely diagnosis given the specific mention of "pes anserine bursitis" in the question. Pes anserine bursitis is an inflammatory condition of the bursa located at the insertion of the sartorius, gracilis, and semitendinosus muscles on the tibia, which is a common cause of medial knee pain.

Medial meniscus tear: This condition can cause medial knee pain and is a common injury in the knee, which might be considered in the differential diagnosis for pes anserine bursitis due to overlapping symptoms.
Medial collateral ligament (MCL) sprain: An MCL sprain can also cause medial knee pain and instability, making it a plausible alternative diagnosis.
Osteoarthritis of the knee: Especially in older patients, osteoarthritis could be a consideration, as it can cause pain and stiffness in the knee, including the medial aspect.

Deep vein thrombosis (DVT): Although less likely, DVT is a critical condition that could present with swelling and pain in the leg, including the area around the knee. Missing this diagnosis could be catastrophic.
Septic arthritis: This is an infection within a joint, which is a medical emergency. While the presentation might differ, any suspicion of septic arthritis warrants immediate investigation.
Tumor: Although rare, a tumor in the soft tissues or bone around the knee could cause persistent pain and should not be overlooked.

Sarcoidosis: This condition can cause inflammation in various parts of the body, including the bursae, and could be a rare cause of pes anserine bursitis-like symptoms.
Gout or pseudogout: These are forms of arthritis that can cause sudden, severe joint pain and could potentially affect the bursae, although this would be an uncommon presentation.
Infection of the bursa (septic bursitis): While infection is a concern in any bursitis, it is less common than inflammatory or traumatic causes and would typically present with significant redness, warmth, and possibly fever.