A 14-year-old boy presents with severe left knee pain exacerbated by running. What is the most likely cause of his condition?

The most likely cause of the 14-year-old boy's severe left knee pain, especially given that it is exacerbated by running, is Osgood-Schlatter disease. This condition is common in adolescents who are active in sports, particularly those involving running and jumping. It occurs due to repetitive stress on the knee, specifically at the tibial tuberosity where the patellar tendon attaches.

As a child grows, the growth plate in this area can become inflamed as a result of regular physical activity, leading to pain and discomfort. The age of the boy and the nature of his symptoms are characteristic of Osgood-Schlatter disease.

In contrast, while chondromalacia and osteochondritis are also conditions that can cause knee pain, they are less common in this age group and context compared to Osgood-Schlatter disease. Ligamental tears, such as those involving the anterior cruciate ligament, might present with acute symptoms and often follow a specific traumatic injury, which does not appear to be the case here. Thus, considering the patient's age, activity level, and symptom profile, Osgood-Schlatter disease is the most fitting diagnosis for his knee pain.