Differential Diagnosis for Acute Knee Pain

The patient presents with sudden onset of severe knee pain, specifically in the back left area, which worsens with weight-bearing activities. The pain began after getting out of a car, with no prior trauma or injury mentioned. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis
  • Bursitis (Prepatellar or Infrapatellar): Given the acute onset of pain, particularly in the back of the knee, bursitis is a strong consideration. Bursae are fluid-filled sacs that cushion bones, tendons, and muscles near joints. Inflammation of the bursae in the knee can cause severe pain, especially with movement or pressure.
• Other Likely Diagnoses
  • Meniscal Tear: A tear in the cartilage of the knee joint can cause pain, especially if the tear is in the posterior horn of the meniscus, which could explain the back knee pain.
  • Ligament Sprain: Although the patient doesn't mention a specific injury, a sudden twist or movement getting out of the car could potentially sprain a ligament, leading to pain and instability.
  • Tendinitis: Inflammation of the tendons around the knee, such as the hamstring or popliteal tendons, could cause pain in the back of the knee.
• Do Not Miss Diagnoses
  • Deep Vein Thrombosis (DVT): While less likely, DVT is a critical diagnosis not to miss due to its potential for severe complications, including pulmonary embolism. The patient's recent period of immobility (e.g., car ride) could be a risk factor.
  • Septic Arthritis: An infection within the joint space, which is a medical emergency requiring prompt treatment to prevent joint destruction.
• Rare Diagnoses
  • Osteochondritis Dissecans: A condition where a piece of cartilage and bone in the joint becomes damaged, leading to pain and swelling. It's more common in younger individuals but could be considered in the differential.
  • Popliteal Cyst (Baker's Cyst): A fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee, which can cause pain, especially if it ruptures.

Each of these diagnoses has different implications for treatment and management, ranging from conservative management with rest, ice, and physical therapy to urgent surgical intervention. A thorough clinical evaluation, including history, physical examination, and potentially imaging studies, is necessary to determine the exact cause of the patient's symptoms.