## Correct Answer: D. MRI sacroiliac joints This clinical presentation is classic for **ankylosing spondylitis (AS)**, a seronegative spondyloarthropathy. The discriminating features are: young male, chronic morning-predominant back pain that improves with activity (inflammatory pattern, not mechanical), and anterior uveitis (extra-articular manifestation of AS). Normal lumbar spine X-ray is crucial—early AS does NOT show spinal changes on plain radiographs. The pathology begins at the **sacroiliac joints**, where inflammatory changes precede spinal involvement by months to years. MRI is the gold standard for detecting early sacroiliitis because it visualizes bone marrow edema and inflammatory changes before bony erosions or sclerosis become radiographically visible. This allows diagnosis at the earliest stage when disease-modifying therapy (DMARDs, TNF inhibitors per Indian guidelines) can prevent progression. Per Harrison and Indian rheumatology practice, MRI sacroiliac joints is the investigation of choice when clinical suspicion is high but plain radiographs are normal. HLA-B27 testing supports diagnosis but does not replace imaging for confirmation. ## Why the other options are wrong **A. Bone scan** — Bone scan (technetium-99m scintigraphy) is non-specific and detects areas of increased osteoblastic activity but cannot differentiate inflammatory from degenerative or infectious processes. It is less sensitive and specific than MRI for early sacroiliitis and does not visualize soft tissue inflammation. In modern practice, bone scan is rarely used for AS diagnosis in India; it has been superseded by MRI. **B. X-ray thoracolumbar spine** — Plain radiographs of the thoracolumbar spine are insensitive in early AS. Radiographic changes (syndesmophytes, squaring of vertebral bodies, bamboo spine) appear only after months to years of disease. The question explicitly states lumbar X-ray is normal, and repeating imaging at a different spinal level will not detect early sacroiliitis. This delays diagnosis and misses the window for early intervention. **C. CT spine** — CT is more sensitive than plain radiography for detecting bony erosions and sclerosis but is still inferior to MRI for detecting early inflammatory changes (bone marrow edema) that precede structural damage. CT delivers higher radiation dose and is reserved for evaluating advanced disease complications. MRI is preferred for early diagnosis in young patients per Indian guidelines. ## High-Yield Facts - **Sacroiliitis is the hallmark of early AS**—inflammation begins at sacroiliac joints before spinal involvement; plain X-rays may be normal for 6–12 months. - **MRI detects bone marrow edema** in sacroiliitis before radiographic changes appear; it is the gold standard for early diagnosis when clinical suspicion is high. - **Morning-predominant back pain improving with activity** is the inflammatory pattern that distinguishes AS from mechanical back pain (which worsens with activity). - **Anterior uveitis is an extra-articular manifestation** of seronegative spondyloarthropathies (AS, reactive arthritis, psoriatic arthritis); occurs in ~25% of AS patients. - **HLA-B27 positivity** supports AS diagnosis but is not diagnostic alone; ~8% of Indian population is HLA-B27 positive without disease. ## Mnemonics **ASIR for early AS diagnosis** **A**nterior uveitis + **S**acroiliitis (MRI) + **I**nflammatory back pain + **R**adiographs normal = early AS. Use when young male with morning pain and uveitis presents with normal plain films. **SAC-IL for sacroiliitis imaging** **S**acroiliac joint → **A**lways **C**hoose **I**maging (MRI) **L**ast (after clinical suspicion + normal X-rays). Reminds that sacroiliac MRI is the final diagnostic step in suspected early AS. ## NBE Trap NBE pairs "normal lumbar X-ray" with "next investigation" to trap students into choosing another radiograph (thoracolumbar X-ray, bone scan, or CT)—all of which miss early sacroiliitis. The trap is assuming that if one spinal region is normal, imaging another region or modality will help; the real answer requires recognizing that sacroiliac joints (not lumbar spine) are the site of early pathology and that MRI (not plain films) detects it. ## Clinical Pearl In Indian clinical practice, young males with inflammatory back pain and uveitis are often initially investigated with lumbar X-rays (which are normal) and then empirically started on NSAIDs. Early MRI of sacroiliac joints can confirm AS diagnosis within weeks, allowing timely initiation of TNF inhibitors (available in India) that can halt disease progression and prevent spinal fusion—a life-altering complication in working-age men. _Reference: Harrison Ch. 328 (Spondyloarthropathies); Robbins Ch. 26 (Musculoskeletal pathology)_
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