Meniscal Injury: The presentation of right knee effusion with persistent pain and swelling, particularly when associated with internal derangement, strongly suggests a meniscal injury. Meniscal tears are common and can cause significant pain and swelling due to the rich innervation of the meniscus and the potential for associated synovitis.

Ligamentous Sprain: Ligament sprains around the knee can lead to effusion and pain, especially if there's an associated instability or if the sprain is severe. The mechanism of injury and physical examination findings can help differentiate this from a meniscal injury.
Synovitis: Inflammation of the synovial lining of the knee joint can cause effusion and pain. This could be primary (e.g., due to rheumatoid arthritis or other autoimmune conditions) or secondary to another condition like a meniscal tear.
Osteochondritis Dissecans: This condition involves a fragment of cartilage and bone becoming detached from the surrounding bone, often due to inadequate blood supply. It can cause knee pain and effusion, especially if the fragment is loose and moving within the joint.

Septic Arthritis: Although less common, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with sudden onset of pain, swelling, and effusion, often accompanied by systemic symptoms like fever.
Tumor (e.g., Synovial Sarcoma): Though rare, tumors affecting the knee can cause chronic pain, swelling, and effusion. A high index of suspicion is necessary, especially in the absence of a clear traumatic or inflammatory cause.
Osteonecrosis: Avascular necrosis of the femoral or tibial condyles can cause knee pain and effusion. It might not be the first consideration but is crucial to diagnose due to its significant impact on joint health and function.

Pigmented Villonodular Synovitis (PVNS): A rare condition characterized by benign growths in the synovium, leading to chronic inflammation and potential bone erosion. It can cause recurrent effusions and is often diagnosed after other conditions have been ruled out.
Hemarthrosis (due to bleeding disorder): In patients with a known or undiagnosed bleeding disorder, hemarthrosis (bleeding into the joint) can occur, leading to acute or recurrent effusions and pain.
Neuropathic Arthropathy (Charcot Joint): Though more commonly associated with the foot, neuropathic arthropathy can affect any joint, including the knee, especially in patients with significant sensory neuropathy. It leads to progressive joint destruction and can mimic other conditions.