How should a 16-year-old female with three months of progressive low back pain, midline bony tenderness from L3 to L5, paraspinal muscle tenderness, calf cramping with straight‑leg raise, normal strength, no paresthesias or bowel/bladder changes, pain worsened by flexion or rotation and improved when lying down, and a past history of pilonidal disease be evaluated and managed?

Immediate MRI of the Lumbar Spine Without Contrast

This 16-year-old requires urgent MRI of the lumbar spine without IV contrast as the first-line imaging study, followed by laboratory workup including inflammatory markers (ESR, CRP) and complete blood count to evaluate for serious pathology including tumor, infection (spondylodiscitis), or stress fracture. 1

Red Flags Present

This patient exhibits multiple concerning features that mandate advanced imaging:

• Progressive pain over 3 months with functional disability (unable to attend school)
• Acute onset without injury in an adolescent
• Midline bony tenderness at L3-L5 (highly specific for internal disk disruption or vertebral pathology) 2
• Bilateral calf cramping with SLR suggesting possible nerve root irritation
• Pain worsened by all flexion/rotation movements indicating significant structural pathology
• Young age (pediatric back pain has higher likelihood of organic pathology than adults) 3

Why MRI Without Contrast First

The 2025 ACR Appropriateness Criteria specifically state that MRI spine without IV contrast is the appropriate first-line test when red flags are present 1. The guideline notes that:

• Targeted or complete spine MRI increases diagnostic yield by 25-34% beyond radiographs alone
• Historically spondylolysis/spondylolisthesis were most common, but intervertebral disk pathology is now more frequently identified due to MRI availability 1
• Precontrast imaging is essential if contrast will be needed later (for suspected tumor or infection)
• MRI evaluates soft tissues, spinal cord, and paraspinal structures that radiographs miss 1

Differential Diagnosis Priority

Given the clinical presentation, consider in order of likelihood and severity:

1. Tumor (primary or metastatic) - progressive pain, bony tenderness, functional disability, young age
2. Spondylodiscitis/osteomyelitis - subacute progressive course, bony tenderness (though typically more systemic symptoms)
3. Disk herniation with nerve root compression - calf cramping with SLR, though bilateral symptoms less typical
4. Spondylolysis/stress fracture - common in adolescents, though usually activity-related
5. Scheuermann kyphosis - less likely given acute onset 3

The midline bony tenderness is particularly concerning: research shows 95.8% of patients with internal disk disruption report midline pain, with 83.5% diagnostic accuracy 2. However, in a 16-year-old with progressive symptoms and functional disability, tumor must be ruled out first.

Laboratory Workup

Obtain simultaneously with imaging:

• ESR and CRP (elevated in infection, some tumors)
• Complete blood count (leukocytosis in infection, anemia in malignancy)
• Consider creatine kinase only if paraspinal compartment syndrome suspected (though this typically follows strenuous exertion and presents with extreme acute pain) 4

Common Pitfalls to Avoid

• Do not start with plain radiographs alone - they miss 25-34% of diagnoses in pediatric back pain with red flags 1
• Do not dismiss as "nonspecific back pain" - while becoming more common in adolescents, this patient's red flags demand investigation 3
• Do not attribute pilonidal disease history to current symptoms - pilonidal disease affects the sacrococcygeal region, not L3-L5, and she has no active flare 5
• Do not delay imaging for "conservative management trial" - 3 months of progressive symptoms with functional disability warrants immediate investigation 6

Contrast Administration Decision

Reserve gadolinium contrast for after reviewing noncontrast images 1. Add contrast if MRI shows:

• Suspected neoplasm (enhancement patterns help characterize)
• Suspected discitis/osteomyelitis (enhancement of disk space and endplates)
• Indeterminate lesions requiring further characterization

The guideline emphasizes that precontrast images are essential to properly assess enhancement 1.