Musculoskeletal System
The musculoskeletal domain tests osteoarthritis, inflammatory arthritis, crystal disease, fractures, low back pain, and osteoporosis, with emphasis on distinguishing mechanical from inflammatory and infectious problems.
Core rules
- Osteoarthritis is activity-related pain with brief morning stiffness and bony enlargement. Treatment is exercise, weight loss, topical or oral NSAIDs, and joint replacement for advanced disease.
- Rheumatoid arthritis is symmetric small-joint inflammation with prolonged morning stiffness. Start a conventional synthetic DMARD such as methotrexate early; do not rely on NSAIDs alone.
- Gout causes acutely painful monoarthritis, commonly the first metatarsophalangeal joint. Negatively birefringent needle-shaped crystals confirm the diagnosis. Treat the flare with NSAIDs, colchicine, or glucocorticoids.
- Osteoporosis is defined by a DEXA T-score of −2.5 or lower or a fragility fracture. Treat with bisphosphonate, denosumab, or anabolic therapy based on risk.
Worked example
A 55-year-old man has an acutely swollen, red first toe. Joint aspiration shows negatively birefringent needle-shaped crystals. This is gout, not septic arthritis. Start an NSAID if renal function is adequate. Urate-lowering therapy is indicated for recurrent attacks or tophi; if started during a flare, continue anti-inflammatory coverage.
Traps
- Missing septic arthritis when one joint is hot and the patient is febrile.
- Treating rheumatoid arthritis with NSAIDs alone and delaying DMARDs.
- Ignoring red flags in back pain: fever, cancer history, neurologic deficit, or saddle anesthesia.
- Prescribing bisphosphonates without checking renal function or calcium.