Correct Answer: B. Isoniazid and rifampicin
Multidrug-resistant (MDR) tuberculosis is defined by the WHO and adopted by India's RNTCP (Revised National TB Control Programme) as resistance to at least isoniazid and rifampicin, the two most potent first-line anti-TB drugs. This definition is critical because isoniazid and rifampicin are the backbone of standard TB therapy—their combination provides synergistic bactericidal activity and is essential for rapid sputum conversion and cure. When resistance develops to both agents, treatment outcomes deteriorate significantly, necessitating prolonged second-line regimens. The RNTCP guidelines (India's TB control standard) explicitly define MDR-TB by this dual resistance, not by additional agents. Fluoroquinolones and injectable drugs like kanamycin are second-line agents used in MDR-TB treatment, but their resistance is not part of the defining criterion. The key discriminator is that MDR-TB is fundamentally about loss of the two most effective first-line drugs; additional resistance patterns (to fluoroquinolones or injectables) define extensively drug-resistant (XDR) TB, a more severe category. Understanding this distinction is essential for Indian medical practice, where RNTCP protocols guide diagnosis, classification, and treatment allocation of TB cases.
Why the other options are wrong
A. Isoniazid, rifampicin, and fluoroquinolone — This is wrong because fluoroquinolone resistance is not part of the MDR-TB definition. Fluoroquinolones are second-line drugs; resistance to them (along with isoniazid and rifampicin) defines XDR-TB (extensively drug-resistant TB), a more severe category. The NBE trap here is conflating MDR with XDR by including a second-line agent in the definition. C. Isoniazid, rifampicin, and kanamycin — This is wrong because kanamycin (an injectable aminoglycoside) is a second-line agent, not part of the MDR-TB definition. Resistance to kanamycin plus isoniazid and rifampicin would indicate pre-XDR or XDR-TB, depending on fluoroquinolone status. The trap is including injectable drug resistance in the MDR criterion, which is incorrect per RNTCP guidelines. D. Fluoroquinolones — This is wrong because fluoroquinolones alone do not define MDR-TB; they are second-line agents. MDR-TB must include resistance to both first-line drugs (isoniazid and rifampicin). Fluoroquinolone resistance alone would not meet the MDR definition. This option tests whether students confuse first-line and second-line drug hierarchies.
High-Yield Facts
- MDR-TB definition: Resistance to at least isoniazid and rifampicin (RNTCP/WHO standard).
- XDR-TB definition: MDR-TB plus resistance to any fluoroquinolone and at least one injectable second-line drug (kanamycin, amikacin, or capreomycin).
- First-line TB drugs: Isoniazid, rifampicin, pyrazinamide, ethambutol (HRZE); resistance to INH+RIF = MDR.
- Second-line TB drugs: Fluoroquinolones, injectables (kanamycin, amikacin), linezolid, bedaquiline; used in MDR-TB treatment.
- India's TB burden: 27 million TB cases globally; India accounts for 23% (RNTCP data); MDR-TB prevalence 2–3% of new cases, 15–20% of retreatment cases.
- Treatment duration: MDR-TB requires 20 months of second-line therapy vs. 6 months for drug-susceptible TB; early detection via CBNAAT/DST is critical.
Mnemonics
MDR = Isoniazid + Rifampicin MDR-TB is defined by resistance to the two most potent first-line drugs: Isoniazid and Rifampicin. Remember: MDR = INH + RIF. If you see fluoroquinolones or injectables in the definition, it's XDR, not MDR. First-line vs. Second-line hierarchy First-line (HRZE): Isoniazid, Rifampicin, pyraZinamide, Ethambutol — used for drug-susceptible TB. Second-line (FIL): Fluoroquinolones, Injectables, Linezolid — used for MDR-TB. MDR definition stops at first-line resistance only.
NBE Trap
NBE pairs MDR-TB with second-line drugs (fluoroquinolones, injectables) to lure students who conflate MDR with XDR. The trap is including agents beyond the two defining first-line drugs (isoniazid and rifampicin) in the MDR criterion.
Clinical Pearl
In Indian TB clinics, CBNAAT (Cartridge-Based Nucleic Acid Amplification Test) detects MDR-TB by identifying rifampicin resistance as a proxy for isoniazid resistance in ~95% of cases. Once MDR-TB is confirmed, patients are enrolled in the RNTCP's Programmatic Management of Drug-Resistant TB (PMDT) scheme, which mandates 20-month second-line therapy—a critical distinction from the 6-month standard regimen.
_Reference: RNTCP Guidelines (India); Harrison Ch. 160 (Tuberculosis); Robbins Ch. 8 (Infectious Diseases)_