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Mrs. K is a 75-year-old woman who complains of a painful left knee.

What is the differential diagnosis of joint pain? How would you frame the differential?

The causes of joint pain range from common to rare and from not particularly dangerous to joint- and life-threatening. Even the most benign causes of joint pain can lead to serious disability. The evaluation of a patient with joint pain calls for a detailed history and physical exam (often focusing on extra-articular findings) and occasionally the sampling of joint fluid and possibly analyzing serologic tests.

There are three pivotal features in organizing the approach to joint pain. First, is the pain articular or extra-articular? Although this distinction may seem obvious, abnormalities of periarticular structures can mimic articular disease. Second, is a single joint or are multiple joints involved? Finally, are the involved joints inflamed or not?

The first pivotal point in making a diagnosis in a patient with joint pain is to determine whether the patient's pain is truly articular, real joint pain, or periarticular.

The differential diagnosis below is organized by these pivotal points: the number of joints involved (monoarticular vs polyarticular) and by whether or not the joint is inflamed (judged by physical exam, joint fluid analysis, or both). Recognize that all of the monoarticular arthritides can present in a polyarticular distribution, and classically polyarticular diseases may occasionally only affect a single joint.

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The joint distribution of diseases that cause joint pain is variable; monoarticular arthritides may present with polyarticular findings and vice versa.

  1. Monoarticular arthritis

    1. Inflammatory

      1. Infectious

        1. Gonococcal arthritis

        1. Nongonococcal septic arthritis

        1. Lyme disease

      1. Crystalline

        1. Monosodium urate (gout)

        1. Calcium pyrophosphate dihydrate deposition disease (CPPD or pseudogout)

    1. Noninflammatory

      1. Osteoarthritis (OA)

      1. Traumatic

      1. Avascular necrosis

  2. Polyarticular arthritis

    1. Inflammatory

      1. Rheumatologic

        1. Rheumatoid arthritis (RA)

        1. Systemic lupus erythematosus (SLE)

        1. Psoriatic arthritis

        1. Other rheumatic diseases

      1. Infectious

        1. Bacterial

          1. Bacterial endocarditis

          1. Lyme disease

        1. Viral

          1. Rubella

          1. Hepatitis B

          1. HIV

          1. Parvovirus

        1. Postinfectious

          1. Enteric

          1. Urogenital

          1. Rheumatic fever

    1. Noninflammatory: OA

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Mrs. K's symptoms started after she stepped down from a bus with unusual force. The pain became intolerable within about 6 hours of onset and has been present for 3 days now. She otherwise feels well. She reports no fevers, chills, dietary changes, or sick contacts.

On physical exam she is in obvious pain, limping into the exam room on a cane. Her vital signs are temperature, 37.0°C; RR, 12 breaths per minute; BP, 110/70 mm Hg; pulse, 80 bpm. The only abnormality on exam is the right knee. It is red, warm to the touch, and tender to palpation. The range of motion is limited to only about 20 degrees.

At this point, what is the leading hypothesis, what are the active alternatives, and is there a ...

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