A 24-year-old male patient presents to your clinic with a 6-week history of anterior knee pain, particularly during activities such as squatting, climbing stairs, and running. The pain has been gradually worsening, and he has not found relief with rest, ice, or over-the-counter pain medications. On physical examination, you note tenderness over the patellar region and crepitus on knee range of motion. Additionally, you observe a slight lateral deviation of the patella during active knee extension. Given the physical examination findings, which muscle should be strengthened to help treat this patient?