Meniscal Tear: Often presents with sudden onset of knee pain, even without a clear history of trauma, due to degenerative changes or minor, unnoticed injuries. The meniscus can tear suddenly, leading to pain and potentially locking or catching sensations in the knee.

Ligament Sprain: Although trauma is a common cause, minor sprains can occur from everyday activities, leading to acute knee pain.
Osteoarthritis: Sudden pain can occur due to a flare-up of osteoarthritis, possibly triggered by overuse or a change in activity level.
Bursitis: Inflammation of the bursae around the knee can cause acute pain, often related to overuse or direct pressure on the knee.
Tendinitis: Inflammation of the tendons around the knee, such as patellar tendinitis, can cause sudden pain, especially with activities that stress the tendon.

Septic Arthritis: Although less common, it is critical to consider septic arthritis, as it is a medical emergency requiring prompt antibiotic treatment to prevent joint destruction.
Spontaneous Hemarthrosis: Bleeding into the joint space can occur without trauma, often due to conditions like hemophilia or anticoagulant use, and requires immediate attention.
Osteonecrosis: Sudden onset of knee pain can be a symptom of osteonecrosis, especially in individuals with risk factors such as long-term corticosteroid use.

Pigmented Villonodular Synovitis (PVNS): A rare condition characterized by benign growths in the synovium, leading to joint pain and swelling.
Synovial Chondromatosis: A rare condition where cartilage grows in the synovium, causing joint pain and swelling.
Knee Hemangioma: Benign vascular tumors that can cause pain and swelling if they occur within the knee joint or surrounding tissues.