## Why "Decreased lung elastic recoil causing lungs to over-expand at functional residual capacity" is right Point **D** marks the functional residual capacity (FRC), the equilibrium point where inward elastic recoil of the lung exactly balances outward elastic recoil of the chest wall, with net respiratory system pressure = 0. In emphysema/COPD, destruction of elastic tissue reduces lung elastic recoil. This means the lungs cannot recoil inward as strongly, so equilibrium is reached at a larger lung volume — point **D** shifts rightward. This manifests clinically as barrel chest and hyperinflation. The patient's decreased elastic recoil (evidenced by reduced PFT values and imaging) is the direct cause of increased FRC. (Guyton & Hall 14e, Ch 38: Lung Compliance and Elastic Properties of the Lungs) ## Why each distractor is wrong - **Increased chest wall stiffness limiting thoracic expansion**: Chest wall stiffness (as in kyphoscoliosis or obesity) would shift point **D** *leftward* (decreased FRC), not rightward. Stiffness opposes outward recoil, reducing the equilibrium volume. - **Increased diaphragmatic muscle strength enhancing inspiratory capacity**: Diaphragmatic strength affects inspiratory reserve volume and total lung capacity, not the FRC equilibrium point. FRC is determined by elastic properties, not muscle strength. - **Acute bronchospasm causing dynamic airway compression during expiration**: Bronchospasm causes air trapping and increased FRC in asthma, but the mechanism is obstruction to flow, not loss of elastic recoil. This does not explain the *decreased elastic recoil* documented in this patient's PFTs. **High-Yield:** FRC shifts rightward in emphysema/COPD (lost elastic recoil → over-expansion) and leftward in pulmonary fibrosis/obesity (stiff lungs/chest wall → under-expansion). [cite: Guyton & Hall 14e Ch 38]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.