Musculoskeletal injury or strain: This is the most common cause of knee and calf pain in children, often resulting from overuse, trauma, or sports injuries. The age and presentation make this a highly plausible initial consideration.

Osgood-Schlatter disease: A common cause of knee pain in children, characterized by inflammation of the patellar tendon at its insertion on the tibia. It typically affects adolescents during growth spurts.
Growing pains: Although more commonly associated with nighttime leg pain, growing pains can occasionally present during the day and might involve the calf and knee areas.
Infection (e.g., osteomyelitis, septic arthritis): While less common, infections can cause severe pain and must be considered, especially if there's fever, redness, or swelling.

Compartment syndrome: A medical emergency that can result from increased pressure within a muscle compartment, leading to nerve and muscle damage. It's crucial to identify and treat promptly.
Malignancy (e.g., osteosarcoma, rhabdomyosarcoma): Although rare, bone and soft tissue cancers can present with pain and must be ruled out, especially with persistent or worsening symptoms.
Vascular issues (e.g., deep vein thrombosis, arterial occlusion): These conditions are rare in children but can be life-threatening and require immediate attention.

Rheumatologic diseases (e.g., juvenile idiopathic arthritis): These conditions can cause joint pain and swelling but are less common and typically have other systemic symptoms.
Neurological conditions (e.g., peripheral neuropathy, radiculopathy): Rare in children, these conditions can cause pain and should be considered if other diagnoses are ruled out.
Inherited disorders (e.g., muscular dystrophy): Certain genetic conditions can lead to muscle weakness and pain, though they are less likely to present acutely with severe knee and calf pain.