Correct Answer: A. 2
Spinal tuberculosis (Pott's disease) commonly leads to psoas abscess formation as the infection tracks along tissue planes from the vertebral bodies. The psoas major muscle (labeled as structure 2 in the CT image) is the primary muscle affected because of its anatomical proximity to the lumbar and thoracic vertebrae and its fascial continuity with the vertebral bodies. The clinical presentation—persistent hip flexion contracture, limited and painful hip motion, and dull backache—is pathognomonic for psoas involvement. The psoas major originates from the lateral surfaces of T12 and L1–L5 vertebrae and intervertebral discs, making it the first muscle to be invaded by tuberculous infection spreading from vertebral osteomyelitis. The flexion contracture occurs because the psoas, when inflamed or abscess-laden, maintains the hip in flexion (the muscle's primary action). On CT imaging, the psoas muscle appears enlarged, with low-attenuation fluid collection (abscess) within or around it. This is a classic complication in Indian TB-endemic regions, where spinal TB remains a significant cause of paravertebral abscess and neurological morbidity. Recognition of psoas involvement is critical for treatment planning—drainage may be required alongside anti-tuberculous therapy.
Why the other options are wrong
B. 1 — Structure 1 typically represents the quadratus lumborum or lateral abdominal wall musculature. While TB can involve adjacent structures, the quadratus lumborum is not the primary muscle affected in Pott's disease. It lacks direct attachment to the vertebral bodies and is not the anatomical pathway for tuberculous spread. This is a distractor that tests knowledge of regional anatomy. C. 3 — Structure 3 likely represents the erector spinae or posterior paraspinal muscles. Although these muscles lie posterior to the vertebral column, they are not the primary site of abscess formation in spinal TB. The infection preferentially tracks anteriorly and laterally along the psoas fascia rather than posteriorly. Posterior muscle involvement is less common and occurs late in disease. D. 4 — Structure 4 probably represents the rectus abdominis or anterior abdominal wall. This structure is too distant from the vertebral bodies to be the primary site of tuberculous involvement. While anterior extension of abscess can occur, the rectus abdominis is not directly attached to the spine and is not the anatomical conduit for infection spread from Pott's disease.
High-Yield Facts
- Psoas major is the primary muscle affected in spinal TB because it originates directly from T12–L5 vertebral bodies and intervertebral discs.
- Psoas abscess presents with hip flexion contracture, limited hip motion, and dull backache—classic clinical triad in TB-endemic regions.
- CT imaging shows enlarged psoas muscle with low-attenuation fluid collection (abscess) within or surrounding the muscle.
- Pott's disease spreads anterolaterally along fascial planes to the psoas, not posteriorly to paraspinal muscles.
- Treatment of psoas abscess in TB requires anti-tuberculous therapy ± percutaneous or surgical drainage depending on abscess size and clinical deterioration.
Mnemonics
PSOAS in TB Pott's disease → Spinal TB → Originates from vertebrae → Abscess forms → Spreads anterolaterally. The psoas major is the anatomical bridge between the spine and hip, making it the first muscle involved in spinal TB. HIP FLEX in Psoas TB Hip flexion contracture, Inflammation of psoas, P = Psoas abscess; Flexion is the action of psoas; Lumbar/thoracic origin; Exudative spread; X = eXtension of infection. Use when you see hip flexion + backache + TB history.
NBE Trap
NBE pairs spinal TB with "backache + hip flexion" to lure students into choosing posterior paraspinal muscles (erector spinae) or lateral muscles (quadratus lumborum), when the discriminating feature is the hip flexion contracture—which only the psoas major can cause due to its unique anatomical action and direct vertebral attachment.
Clinical Pearl
In Indian TB-endemic settings, a young patient presenting with spinal TB and a fixed hip flexion posture should immediately raise suspicion for psoas abscess. This finding often mandates imaging and may require drainage before the patient develops neurological deficit from spinal cord compression or sepsis from abscess rupture.
_Reference: Robbins Ch. 8 (Infectious Diseases); Bailey & Love Ch. 34 (Tuberculosis of Bones and Joints); Harrison Ch. 165 (Tuberculosis)_