Which physiological principle best explains why deficiency of pulmonary surfactant leads to diffuse alveolar collapse in respiratory distress syndrome?

• A By the Young-Laplace relationship, smaller alveoli develop greater collapsing pressure, and surfactant normally offsets this by lowering surface tension more in small alveoli ✓
• B Loss of surfactant increases alveolar wall permeability, flooding alveoli with protein-rich fluid
• C Surfactant deficiency causes bronchiolar smooth muscle spasm, raising airway resistance and promoting air trapping
• D Absence of surfactant abolishes hypoxic pulmonary vasoconstriction, creating right-to-left shunting through the ductus arteriosus

Explanation

The Young-Laplace equation states that transpulmonary pressure required to keep an alveolus open is proportional to surface tension divided by radius, so small alveoli tend to collapse into larger ones. Surfactant lowers surface tension proportionately more as alveolar size falls, stabilizing alveoli of different sizes. Without it, small alveoli collapse at end expiration, causing atelectasis, reduced compliance and the ground glass radiographic pattern. Altered vascular reactivity and bronchospasm are not primary mechanisms of surfactant deficiency.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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