Ans. is 'a' i.e., Isoniazid + Rifampicin * Multidrug-resistant TB (MDR-TB) are by definition infected with strains of Mycobacterium tuberculosis resistant to isoniazid and rifampin.* Extremely drug resistant TB (XDR-TB) due to MDR strains that are resistant to all fluoroquinolones and to at least one of three second-line injectable agents (amikacin, kanamycin, and capreomycin).* Following are definitions of drug resistant tuberculosis :Definitions of drug-resistant tuberculosis51* Drug-resistant tuberculosis: tuberculosis caused by an isolate of mycobacterium tuberculosis that is resistant to least 1 of the first-line antituberculosis drugs: isoniazid, rifampin, ethambutol, pyrazinamide, or streptomycin* Monoresistance: resistance to 1 antituberculosis medication.* Multidrug-resistant tuberculosis: tuberculosis caused by an isolate of M tuberculosis that is resistant to the 2 most effective first-line drugs-isoniazid and rifampin-and possibly other antituberculosis drugs.* Extensively drug- resistant tuberculosis: multidrug-resistant tuberculosis that is also resistant to the most effective second-line drugs, fluoro-quinolones, and at least 1 of the 3 injectable second-line drugs used to treat tuberculosis (amikacin, kanamycin, or capreomycin) and, possibly, to other antituberculosis drugs.