Pulmonary surfactant prevents alveolar collapse by reducing surface tension

Steps:

• Surface tension in alveolar fluid creates inward pressure via Laplace's law (P = 2T/r), risking collapse in small alveoli.
• Pulmonary surfactant, produced by type II cells, lowers surface tension as alveoli shrink.
• Evaluate statements: 1 likely misstates mechanism (e.g., increases tension); 2 confuses with another factor (e.g., elasticity); 3 correctly identifies surfactant's role.
• Only statement 3 directly prevents collapse by decreasing T in Laplace's law.

Why D is correct:

• Surfactant reduces surface tension (T), countering collapse per Laplace's law (P = 2T/r).

Why the others are wrong:

• A, B, C include 1 or 2, which do not address surface tension reduction.

Final answer: D