What is the cause of left knee swelling and pain on ambulation in a 7-year-old female with no known trauma, no fever, and no ecchymosis (bruising)?

Juvenile Idiopathic Arthritis (JIA): This is a common cause of knee pain and swelling in children, especially without a history of trauma. JIA can present with joint pain, swelling, and stiffness, often without systemic symptoms like fever.

Osgood-Schlatter Disease: A common cause of knee pain in children, typically affecting the tibial tubercle. It's often associated with activities that involve running or jumping.
Patellofemoral Pain Syndrome: Characterized by pain around or behind the patella, usually related to activities that put stress on the knee, such as walking, running, or climbing stairs.
Meniscal or Ligamentous Injury: Even without a clear history of trauma, minor injuries can occur, leading to meniscal or ligamentous damage, which might not always present with bruising or fever.

Septic Arthritis: Although the patient does not have fever, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with joint pain and swelling, and the absence of fever does not rule it out, especially in young children.
Osteomyelitis: An infection of the bone that can cause pain, swelling, and sometimes fever. Early diagnosis is crucial to prevent long-term damage.
Tumor (e.g., Osteosarcoma, Ewing's Sarcoma): Although rare, bone tumors can present with pain and swelling, and it's critical to consider them to ensure timely diagnosis and treatment.

Hemophilia: If the child has an undiagnosed bleeding disorder like hemophilia, even minor trauma could lead to significant joint bleeding and swelling.
Sickle Cell Disease: Patients with sickle cell disease can experience vaso-occlusive crises that lead to joint pain and swelling due to sickling of red blood cells within the joint.
Rheumatic Fever: A complication of untreated streptococcal infection, which can cause joint pain and swelling among other symptoms.