Osteoarthritis (OA) of the left knee: The symptoms of morning knee pain that improves with activity, crepitus, slight limitation of knee motion, and increased pain with weight-bearing activities are classic for OA. The fact that it's localized to one knee also fits with OA, as it can be asymmetric.

Meniscal tear: The presence of crepitus and pain with weight-bearing activities could suggest a meniscal tear, especially if there was a history of trauma or twisting injury. However, the absence of locking or catching symptoms makes this less likely.
Patellofemoral pain syndrome (PFPS): This condition is characterized by pain around or behind the patella, especially with activities that load the patellofemoral joint, such as climbing stairs or squatting. While it could explain some of the symptoms, the presence of crepitus and limitation of motion is less typical for PFPS.
Ligamentous sprain: A sprain, particularly of the medial collateral ligament, could cause pain and limitation of motion. However, the morning stiffness and improvement with activity are not as typical for a ligamentous injury.

Septic arthritis: Although less likely given the chronic nature of the symptoms and the absence of systemic symptoms like fever, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. Any suspicion of infection, especially in an immunocompromised patient or one with a history of recent joint injection or surgery, warrants immediate attention.
Tumor or cancer: Bone or soft tissue tumors around the knee could cause pain, limitation of motion, and other symptoms. While rare, missing a diagnosis of cancer could have devastating consequences.
Osteonecrosis (avascular necrosis): This condition, which can be caused by various factors including steroid use, alcohol abuse, or trauma, leads to the death of bone tissue due to lack of blood supply. It can present with knee pain and limitation of motion, especially if the femoral condyle is involved.

Rheumatoid arthritis (RA): Although RA can cause knee pain and stiffness, it typically involves multiple joints and has other systemic symptoms. The asymmetric involvement and lack of other joint symptoms make this less likely.
Gout or pseudogout: These conditions can cause acute, severe knee pain and swelling. However, they usually present with more dramatic symptoms and may not fit as well with the chronic, activity-related pain described.
Neuropathic pain: Rarely, neuropathic pain from a nerve injury or compression could cause knee pain. However, this would typically have a different character (e.g., burning, tingling) and distribution.