This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Gram Positive Bacilli, Gram Negative Bacilli), Subject name: Microbiology
[{'content': 'Resistance to isoniazid, the likely mechanism for resistance is \nSelect and explain. \nA. b-Lactamase, B. Mutations in the katG gene, C. Alterations in the rpoB gene, D. Mutations in the DNA gyrase gene', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 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.', 'role': 'assistant'}]