A 28-year-old woman from Delhi presents with a 2-month history of cough, low-grade fever, and weight loss. She is a healthcare worker with recent exposure to a TB patient. Sputum smear microscopy (Ziehl-Neelsen stain) is negative on two occasions, but a culture on Middlebrook 7H10 agar grows small, buff-coloured colonies after 4 weeks. The isolate is niacin-positive, nitrate-positive, and catalase-positive (heat-labile). A subsequent drug susceptibility test shows resistance to isoniazid and rifampicin. Which of the following is the MOST appropriate next step in the management of this patient?