## Why Supine versus erect spirometry is right The pattern marked **A** — reduced TLC with preserved FEV1/FVC ratio and NORMAL DLCO — is pathognomonic for extra-pulmonary restriction. In this patient with kyphoscoliosis, the preserved DLCO (92% predicted) already excludes parenchymal lung disease. The critical next step is supine versus erect spirometry: a drop in FVC ≥25% when supine (compared to erect) indicates diaphragmatic weakness or mechanical disadvantage from the chest wall deformity, confirming the extra-pulmonary localization. This positional change is a hallmark of extra-pulmonary restriction and guides management (e.g., non-invasive ventilation). [Harrison 21e Ch 286; ATS/ERS 2022] ## Why each distractor is wrong - **Measurement of MIP/MEP**: While MIP and MEP quantify respiratory muscle strength and are useful in neuromuscular disease, they are NOT the most appropriate NEXT step. The clinical presentation (kyphoscoliosis with preserved DLCO) already strongly suggests extra-pulmonary restriction; supine FVC is more specific for confirming mechanical disadvantage of the chest wall or diaphragm. - **HRCT chest with fibrosis protocol**: HRCT is unnecessary here. The DLCO of 92% predicted rules out parenchymal/interstitial lung disease. HRCT would only be indicated if DLCO were reduced, which it is not. Ordering HRCT would delay diagnosis and waste resources. - **Transbronchial lung biopsy**: Biopsy is contraindicated and inappropriate. The preserved DLCO excludes interstitial lung disease, making biopsy unnecessary and potentially harmful in a patient with severe restrictive physiology. **High-Yield:** Restriction + preserved DLCO = extra-pulmonary; restriction + reduced DLCO = parenchymal. Supine FVC drop ≥25% confirms diaphragmatic/chest wall mechanical limitation. [cite: Harrison 21e Ch 286; ATS/ERS Standardisation of Lung Function Testing 2022]
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