Q. A 10-year-old boy has been brought to the physician's office for gradually worsening anterior knee pain on the left side for the past 6 months. The pain was mild to start with but slowly he started limping and he hesitates to participate in school sports. The pain increases on activities like running, climbing stairs, squatting, and kneeling and it is completely relieved by taking rest. His temperature is 98.6°F, heart rate is 86 beats per minute, blood pressure is 100/70 mm of Hg and respiratory rate is 14 per minute. A physical examination of the left knee shows a prominent bony tibial tubercle with focal tenderness and shortened hamstrings with taut quadriceps. The range of motion of the knee is normal and pain is reproduced on extension against resistance. The hip joint looks normal and the straight-leg raising test is painless. The rest of the examination is normal. What is the most likely diagnosis for this patient based upon his clinical features?