Differential Diagnosis

The patient's symptoms of back pain with morning stiffness, bilateral knee and ankle pain, limited range of motion of the back, bilateral knee arthritis, and tenderness over the Achilles tendon, along with a high ESR, suggest an inflammatory condition. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Ankylosing Spondylitis (A): This condition is characterized by chronic inflammation of the spine and sacroiliac joints, leading to severe, chronic pain and discomfort. The patient's back pain with morning stiffness and limited range of motion of the back are key indicators. Ankylosing spondylitis often involves other joints and can cause Achilles tendonitis, which aligns with the patient's symptoms.

• Other Likely Diagnoses

  • Rheumatoid Arthritis (B): Although more commonly associated with small joint involvement, rheumatoid arthritis can cause symptoms similar to those described, including morning stiffness and elevated ESR. However, the prominent back involvement and Achilles tendon tenderness are less typical.
  • Seronegative Rheumatoid Arthritis (C): This diagnosis refers to rheumatoid arthritis without the presence of rheumatoid factor. It could be considered if the patient's symptoms and lab results align with rheumatoid arthritis but lack serological evidence.

• Do Not Miss Diagnoses

  • Infectious Spondylitis or Septic Arthritis: Although less likely given the chronic nature of the symptoms, infectious causes of back pain and arthritis must be considered due to their potential severity and need for urgent treatment. A high ESR can be seen in infections, making this a critical differential diagnosis to rule out.
  • Malignancy (e.g., Multiple Myeloma): Certain malignancies can cause bone pain, elevated ESR, and joint symptoms. While less common, these conditions are critical to diagnose early.

• Rare Diagnoses

  • Psoriatic Arthritis: This condition can cause a combination of back pain, peripheral arthritis, and enthesitis (inflammation of the sites where tendons or ligaments insert into the bone), which could explain the Achilles tendon tenderness. However, the absence of skin psoriasis in the description makes this less likely.
  • Reactive Arthritis: Previously known as Reiter's syndrome, this form of arthritis occurs as a reaction to an infection elsewhere in the body, often in the gastrointestinal tract or the urethra. It can cause back pain, arthritis, and enthesitis but typically follows a distinct infectious episode.
  • Enteropathic Arthritis: Associated with inflammatory bowel diseases like Crohn's disease and ulcerative colitis, this condition can cause peripheral arthritis, back pain, and elevated inflammatory markers. However, gastrointestinal symptoms are usually prominent, which are not mentioned in the scenario.