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Mechanical low back pain — USMLE Step 3 MCQ

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HardAmbulatory Medicine / Primary CareMechanical low back painUSMLE Step 3

A patient with metastatic prostate cancer develops severe lumbar pain, bilateral leg weakness, saddle anesthesia, and urinary retention over 6 hours. MRI is immediately available, but the patient previously had severe anaphylaxis to a gadolinium-based contrast agent. Which initial imaging study should be obtained now?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Urgent noncontrast MRI of the lumbar spine for neurosurgical operative planning

The best answer is “Urgent noncontrast MRI of the lumbar spine for neurosurgical operative planning”. Rapid bilateral weakness, saddle anesthesia, and retention indicate cauda equina compression. ACR rates both lumbar MRI without contrast and MRI without-and-with contrast as Usually Appropriate initial studies; the severe gadolinium reaction makes the noncontrast MRI protocol the uniquely appropriate MRI choice here. CT and CT myelography are less-preferred alternatives when diagnostic MRI cannot be performed, not substitutes merely because gadolinium is contraindicated.

Reference: ACR Appropriateness Criteria: Low Back Pain: https://acsearch.acr.org/docs/69483/narrative/