A 32-year-old man from Delhi presents with a 4-month history of productive cough, fever, and weight loss. Chest X-ray shows left upper lobe infiltrates. Sputum smear microscopy is positive for acid-fast bacilli. He was treated with standard first-line anti-TB drugs (HRZE) for 2 months but showed no clinical improvement. Drug susceptibility testing confirms resistance to both isoniazid and rifampicin. What is the drug of choice for initiating treatment in this patient with MDR-TB?