A 58-year-old woman with seropositive rheumatoid arthritis on weekly methotrexate 17.5 mg for 6 months presents with subacute non-productive cough, fever, and progressive dyspnea on exertion over 3 weeks. SpO₂ is 90% on room air. HRCT shows bilateral ground-glass opacities and patchy consolidation. Spirometry reveals the pattern marked **A** in the diagram. Which of the following is the most appropriate IMMEDIATE management step?
A 58-year-old woman with a 9-year history of seropositive rheumatoid arthritis on weekly methotrexate 15 mg presents with 6 weeks of progressive dry cough and dyspnea on exertion. HRCT shows bilateral ground-glass opacities with basal predominance and centrilobular nodules. Bronchoalveolar lavage demonstrates lymphocytic alveolitis with markedly reduced CD4/CD8 ratio. Pulmonary function testing is performed. The parameter marked **C** in the diagram (DLCO 48% predicted) is markedly reduced, while FEV1/FVC ratio remains preserved at 0.82. Which of the following best explains the clinical significance of the disproportionately reduced DLCO in this patient?
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