If rifampin resistance is detected by rapid molecular testing, which condition is MDR-TB most likely?

• A. Extensively drug-resistant TB
• B. Drug-susceptible TB
• C. MDR-TB
• D. Mono-resistance to isoniazid

Answer: (C)

Rifampin resistance detected by rapid molecular testing is used as a strong signal for MDR-TB because these rapid tests (like GeneXpert) identify mutations that confer resistance to rifampin, and in most settings rifampin resistance coincides with isoniazid resistance. Since MDR-TB is defined as resistance to at least rifampin and isoniazid, finding rifampin resistance makes MDR-TB the most likely diagnosis. Extensively drug-resistant TB would require additional resistance to fluoroquinolones and a second-line injectable, which is not implied by rifampin resistance alone. Mono-resistance to isoniazid would mean resistance to INH with susceptible rifampin, which contradicts rifampin resistance. In rare cases, rifampin resistance can occur without INH resistance, but the established practice uses rifampin resistance as a proxy for MDR-TB due to the high likelihood of concurrent INH resistance.