Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella, often associated with activities that put stress on the knee, like running. The presence of patella pain and crepitis (grinding sensation) on examination strongly supports this diagnosis, as it is commonly seen in PFPS due to the abnormal tracking of the patella within the femoral groove.

Chondromalacia Patellae: This condition involves the softening of the cartilage on the underside of the patella, leading to knee pain and crepitis. It is a common cause of knee pain in adolescents and young adults, especially those who are active in sports.
Patellar Tendinopathy: Also known as jumper's knee, this condition involves inflammation of the tendon that connects the patella to the tibia. It is common in athletes who participate in sports that involve jumping, and can cause pain and stiffness around the patella.
Osgood-Schlatter Disease: This is a condition that affects the upper tibia, where the patellar tendon attaches. It is common in adolescents and can cause knee pain, especially after running or jumping.

Fracture (e.g., Patellar or Tibial Fracture): Although less likely given the symptoms, a fracture must be considered, especially if there was significant trauma or if the patient reports hearing a "pop" at the time of injury. Missing a fracture could lead to improper healing and long-term disability.
Ligamentous Injury (e.g., ACL or MCL Tear): While the symptoms described do not directly suggest a ligamentous injury, such injuries can occur with knee trauma and must be considered to avoid missing a potentially serious condition that requires prompt intervention.
Osteonecrosis: Although rare in this age group, osteonecrosis (death of bone tissue due to lack of blood supply) can occur and must be considered, especially if the patient has risk factors such as steroid use or sickle cell disease.

Patellar Dislocation: This is a condition where the patella slips out of place, usually laterally. It can cause severe pain and is often associated with a traumatic event.
Tumor (e.g., Osteosarcoma): Bone tumors are rare but can cause knee pain and should be considered in the differential diagnosis, especially if the pain is persistent, worsening, or accompanied by other systemic symptoms like weight loss or fever.