A 42-year-old man from rural India presents with a 6-month history of progressive back pain, low-grade fever, and night sweats. Plain radiograph of the thoracolumbar spine shows a sharp angular kyphotic deformity. MRI reveals marrow edema in two adjacent vertebral bodies with preservation of the intervertebral disc space and a paravertebral collection. The structure marked **A** in the diagram (anterior wedging/collapse of vertebral bodies) is the result of which pathophysiologic mechanism in Pott disease?
A. Acute inflammatory osteomyelitis with rapid necrosis of the entire vertebral body and loss of structural integrity
B. Direct invasion of the intervertebral disc by mycobacteria leading to rapid disc destruction and secondary vertebral body collapse
C. Hematogenous spread of Mycobacterium tuberculosis to the anterior inferior vertebral body with subsequent erosion of endplates and progressive vertebral collapse
D. Pathologic fracture due to osteoporosis from chronic malnutrition and malabsorption in tuberculosis
Explanation
Why option 1 is correct
Pott disease (tuberculous spondylitis) is spinal tuberculosis caused by hematogenous spread of Mycobacterium tuberculosis to the vertebral bodies, accounting for ~50% of skeletal TB. The infection typically begins in the anterior inferior vertebral body (well-vascularized subchondral region), spreads beneath the anterior longitudinal ligament, and erodes the endplates. This progressive anterior vertebral collapse produces the characteristic gibbus deformity (sharp angular kyphosis) seen in this patient's imaging. The disc is initially spared because it is avascular and lacks the enzymatic machinery for mycobacterial invasion — disc narrowing occurs later as endplates collapse. This disc-sparing pattern early in disease is a key distinguishing feature of TB spondylitis (WHO TB Guidelines; Harrison's 21e).
Why each distractor is wrong
Option 2: Pyogenic spondylodiscitis (not TB) is characterized by rapid disc destruction with early disc space narrowing. In Pott disease, the disc is initially spared and narrowing occurs late — this is a cardinal distinguishing feature. The question's imaging explicitly shows preservation of the intervertebral disc space, ruling out this mechanism.
Option 3: Acute inflammatory osteomyelitis describes pyogenic infection, not tuberculosis. TB is a chronic, indolent infection with granulomatous inflammation and caseating necrosis, not acute suppuration. The clinical timeline (6 months of insidious symptoms) is inconsistent with acute osteomyelitis.
Option 4: While malnutrition may contribute to bone health, the anterior vertebral collapse in Pott disease is specifically due to mycobacterial destruction of the vertebral endplates and bone, not osteoporotic fracture. This mechanism does not explain the characteristic gibbus deformity or the imaging findings.
High-YieldNEET PG
In Pott disease, the intervertebral disc is spared early (avascular, no mycobacterial enzymes) — disc narrowing occurs late as endplates collapse. This disc-sparing pattern distinguishes TB spondylitis from pyogenic spondylodiscitis.
WHO TB Guidelines; Harrison's Principles of Internal Medicine, 21e; Indian Tuberculosis Programme NTEP
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