Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.
A 69-year-old woman is evaluated for a 5-day history of pain and swelling in the right knee that developed after a prolonged period of gardening in her backyard. Her pain worsens when she walks and is relieved with rest. She has no morning stiffness.
On physical examination, temperature is normal, blood pressure is 128/72 mm Hg, pulse rate is 88/min, and respiration rate is 17/min. BMI is 32. The right knee has a moderate effusion but is not warm or erythematous. Range of motion of the right knee elicits pain. The remainder of the musculoskeletal examination is normal.
Plain radiographs of the right knee show osteophytes and medial joint-space narrowing. Arthrocentesis is performed, and synovial fluid analysis reveals 250 leukocytes/µL (88% lymphocytes, 12% macrophages).
Which of the following is the most likely diagnosis?
A) Bacterial arthritis
B) Crystal-induced arthritis
C) Fungal arthritis
D) Osteoarthritis
MKSAP Answer and Critique
The correct answer is D) Osteoarthritis. This item is available to MKSAP 15 subscribers as item 10 in the Rheumatology section.
This patient most likely has osteoarthritis of the knee. Osteoarthritis may manifest as acute monoarticular arthritis and is usually noninflammatory. This condition is characterized by pain on activity that is relieved with rest and most commonly involves the proximal and distal interphalangeal and first carpometacarpal joints of the hands as well as the weight-bearing joints such as the knees and hips. Range of motion of the involved joint in patients with osteoarthritis is usually painful and may be limited. This patient’s radiographic findings of joint-space narrowing and osteophytes are consistent with osteoarthritis. The synovial fluid leukocyte count in patients with osteoarthritis is typically between 200 and 2,000/µL and may be associated with a predominance of lymphocytes.
Bacterial, crystal-induced, and fungal arthritis also may manifest as monoarticular arthritis. However, these conditions typically are associated with inflammation and would typically cause morning stiffness, warmth, and erythema of the involved joint. Bacterial and fungal arthritis would most likely be associated with a synovial fluid leukocyte count of at least 10,000/µL with a predominance of neutrophils, and the synovial fluid leukocyte count in patients with bacterial arthritis often exceeds 100,000/µL. Finally, bacterial and crystal-induced arthritis often have an acute presentation, whereas fungal arthritis is usually chronic in nature at the time of presentation.
Key Point
- The acute presentation of a noninflammatory monoarthritis involving a weight-bearing joint is suggestive of osteoarthritis.
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