Traumatic knee injury (e.g., ligament sprain, meniscal tear): This is the most likely diagnosis given the acute presentation of fluid on one knee, which could be due to a recent injury causing inflammation and effusion.

Osteoarthritis: Although less common in a 30-year-old, osteoarthritis could be a consideration, especially if there's a history of previous knee injuries or surgeries.
Infectious arthritis: Bacterial, viral, or fungal infections can cause knee effusion, and while less common, should be considered, especially if there are systemic symptoms like fever.
Gout or pseudogout: These crystalline arthropathies can cause sudden onset of knee pain and effusion, particularly in individuals with a history of similar episodes or risk factors like hyperuricemia.

Septic arthritis: This 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 signs of infection like fever and chills.
Hemarthrosis due to bleeding disorder or anticoagulation: In individuals with bleeding disorders (e.g., hemophilia) or those on anticoagulant therapy, even minor trauma can lead to significant bleeding into the joint, necessitating urgent evaluation and management.
Tumor or malignancy: Although rare, primary or metastatic tumors can cause knee effusion and should be considered, especially if there's a history of cancer or if the effusion is persistent and unexplained by other causes.

Rheumatoid arthritis or other autoimmune arthropathies: While these conditions can cause knee effusion, they typically present with symmetric joint involvement and systemic symptoms.
Hemophilic arthropathy: In patients with hemophilia, recurrent bleeding into joints can lead to chronic arthropathy, which might present with effusion among other symptoms.
Pigmented villonodular synovitis (PVNS): A rare condition characterized by benign proliferation of the synovium, leading to chronic knee effusion and potentially causing bone erosion and cysts.