**Core Concept**
Osgood-Schlatter disease is a common cause of knee pain in adolescents, particularly during periods of rapid growth and increased physical activity. This condition is characterized by inflammation of the patellar tendon at its insertion on the tibial tubercle, leading to pain and swelling in the affected area.

**Why the Correct Answer is Right**
The patient's symptoms, including progressive knee pain and a swollen tibial tubercle, are classic for Osgood-Schlatter disease. This condition is often seen in adolescents who participate in sports that involve running, jumping, or repetitive knee bending, such as basketball. The inflammation of the patellar tendon at its insertion on the tibial tubercle is thought to be caused by the repetitive stress and strain on the tendon, leading to micro-tears and inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** (not needed as it's the correct answer)
**Option B:** A popliteal cyst, also known as a Baker's cyst, is a fluid-filled swelling that occurs at the back of the knee. While it can cause knee pain and swelling, it is not typically associated with a swollen tibial tubercle.
**Option C:** Slipped capital femoral epiphysis (SCFE) is a hip condition that occurs in adolescents, typically during periods of rapid growth. It is characterized by the displacement of the femoral head from its normal position, leading to hip pain and limited mobility. SCFE is not typically associated with knee pain or a swollen tibial tubercle.
**Option D:** Legg-Calve-Perthes disease is a hip condition that occurs in children, typically between the ages of 4 and 8. It is characterized by the necrosis of the femoral head, leading to hip pain and limited mobility. Like SCFE, Legg-Calve-Perthes disease is not typically associated with knee pain or a swollen tibial tubercle.

**Clinical Pearl / High-Yield Fact**
Osgood-Schlatter disease is a self-limiting condition that typically resolves on its own within 12-18 months. Treatment focuses on pain management and activity modification, such as avoiding activities that exacerbate the condition and using ice and compression to reduce inflammation.

**✓ Correct Answer: A. Osgood-Schlatter disease**