Resistance to isoniazid, the likely mechanism for resistance is

Strains of M. tuberculosis resistant to individual drugs arise by spontaneous point mutations in the mycobacterial genome that occur at low but predictable rates (10-7-10-10for the key drugs). Resistance to rifampin is associated with mutations in the rpoB gene in 95% of cases; That to isoniazid with mutations mainly in the katG (50-95% of cases) and inhA (up to 45%) genes; That to pyrazinamide in the pncAgene (up to 98%); That to ethambutol in the embB gene (50-65%). Because there is no cross-resistance among the commonly used drugs, the probability that a strain will be resistant to two drugs is the product of the probabilities of resistance to each drug and thus is low.