Musculoskeletal — USMLE Step 2 CK Notes

The one question to ask first

Every MSK stem resolves faster if you decide early whether the pain is inflammatory or mechanical.

The hot joint

A single hot, swollen joint is septic arthritis until aspirated. Fever is present in only about half of cases, so its absence proves nothing. Aspirate before antibiotics unless the patient is frankly septic.

• Synovial WBC over 50,000/µL with neutrophil predominance → infection
• Negatively birefringent needles → gout  ·  positively birefringent rhomboids → pseudogout
• Crystals do not exclude infection. They coexist more often than students expect

Joint pattern recognition

• Symmetrical MCP/PIP/wrist, DIP spared → rheumatoid arthritis
• DIP and PIP nodes, knees, hips, first CMC → osteoarthritis
• First MTP, sudden overnight → gout
• Knee or wrist in an older patient, chondrocalcinosis → pseudogout
• Girdle pain, age over 50, normal CK → polymyalgia rheumatica

Three emergencies you cannot miss

Compartment syndrome

Pain out of proportion, escalating despite opioid, worst on passive stretch. Pulses are usually present — their presence never excludes it. Remove the cast to skin, keep the limb at heart level (not elevated), and go to fasciotomy.

Cauda equina syndrome

Bilateral sciatica, saddle anaesthesia, painless retention with overflow, reduced anal tone. Urgent MRI, then decompression within 24–48 hours.

Giant cell arteritis

Age over 50, new temporal headache, jaw claudication, very high ESR. Steroids first, biopsy second. Vision lost is not recovered.

Bone metabolism

DEXA T-score ≤ −2.5 is osteoporosis. Screen women from 65, earlier with risk factors. A fragility fracture is diagnostic regardless of the DEXA result. Bisphosphonates are taken upright and fasting with water.