The clue is that imaging has reached its limit. In a 70-year-old with prior pulmonary TB on ATT, a newly found vertebral lesion on MRI raises two competing possibilities: tuberculous spondylitis versus a malignant lesion such as metastasis or myeloma. These cannot be reliably separated by appearance alone.
The principle in oncology and infectious-disease workup is the same: when a focal destructive lesion's nature is uncertain, obtain tissue. A CT-guided or open biopsy provides histopathology plus material for AFB culture and sensitivity, simultaneously confirming or excluding TB and screening for cancer in one step.
By contrast, a Mantoux merely reflects past exposure (uninformative here), electrophoresis is a non-specific myeloma screen, and a CT chest evaluates the wrong region. None gives a definitive vertebral diagnosis.
\[\boxed{\text{Biopsy} - \text{tissue diagnosis is the definitive next investigation}}\]