## Why option 1 is correct The scooped or concave expiratory limb (pattern **B**) is the hallmark of obstructive lung disease. This occurs because loss of elastic recoil and small airway obstruction cause dynamic compression of airways during forced expiration. As intrathoracic pressure increases, the smaller airways collapse preferentially, limiting flow rate in the mid-to-late expiratory phase. This creates the characteristic "scoop" shape. The patient's FEV1/FVC < 0.70, elevated TLC, and elevated RV (air trapping) confirm obstructive physiology consistent with COPD (Harrison 21e Ch 280; Guyton & Hall 14e Ch 38). ## Why each distractor is wrong - **Option 2**: This describes pattern **C** (restrictive disease), which shows a small but normally shaped loop. Restrictive disease preserves the FEV1/FVC ratio (>0.70) and reduces all volumes proportionally, not the scooped expiratory pattern seen in pattern **B**. - **Option 3**: This describes pattern **D** (fixed upper airway obstruction), which flattens both inspiratory and expiratory limbs symmetrically. The patient's post-bronchodilator spirometry and air trapping are inconsistent with fixed obstruction. - **Option 4**: Obstructive disease characteristically shows DECREASED FEF25-75 (small airway obstruction), not normal or increased values. Peak expiratory flow may be reduced due to airway collapse, not preserved. **High-Yield:** Scooped expiratory limb = dynamic airway compression in obstructive disease; small proportional loop = restrictive disease; flattened both limbs = fixed upper airway obstruction. [cite:Harrison 21e Ch 280; Guyton & Hall 14e Ch 38]
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