Osteosarcoma or other bone tumor: The elevated alkaline phosphatase (alk phos) level, which is significantly above the normal range, combined with the patient's age and symptoms of femur and knee pain, suggests a bone-related issue. Osteosarcoma is a common primary bone malignancy in adolescents, and the elevated alk phos supports this possibility, as it is often associated with bone formation and turnover.

Osteoid Osteoma: A benign bone tumor that can cause pain, especially at night, and is often associated with elevated alk phos levels. The lack of significant findings on the X-ray does not rule out this diagnosis, as osteoid osteomas can be small and not visible on initial X-rays.
Stress Fracture: Although the X-ray did not show an acute fracture, stress fractures might not be immediately visible. The patient's symptoms and the normal appearance of the X-ray could still be consistent with a stress fracture, especially if the patient has recently increased physical activity.
Osgood-Schlatter Disease: A common cause of knee pain in adolescents, characterized by inflammation of the patellar tendon at its insertion on the tibia. It's often associated with growth spurts and increased physical activity.

Ewing's Sarcoma: Another type of bone cancer that can present with similar symptoms to osteosarcoma. It's crucial to consider this diagnosis due to its aggressive nature and the need for prompt treatment.
Infection (Osteomyelitis or Septic Arthritis): Although the labs and X-ray do not strongly suggest an infection at this point, it's a critical diagnosis not to miss due to its potential for serious complications if not treated promptly.
Hyperparathyroidism: The elevated calcium level could suggest primary hyperparathyroidism, a condition that can lead to bone resorption and pain. This diagnosis is less likely given the patient's age but should be considered due to its potential impact on bone health.

Fibrous Dysplasia: A rare condition where normal bone is replaced with fibrous tissue, leading to weakened bones and potential for pain and fractures.
Hypophosphatasia: A rare genetic disorder characterized by deficient activity of alkaline phosphatase in the body, leading to bone softening. The presentation can vary, but the elevated alk phos in this case makes it less likely.
Gaucher's Disease: A genetic disorder that can lead to bone pain and other systemic symptoms due to the accumulation of glucocerebroside in cells. It's rare and would be an unusual presentation in this context.