**Question:** Surfactant deficiency causes -

A. Respiratory distress syndrome (RDS)
B. Pulmonary fibrosis
C. Congenital hyaline membrane disease (HMD)
D. Pulmonary emphysema

**Core Concept:** Surfactant is a mixture of lipids and proteins that plays a crucial role in maintaining the stability of alveoli, preventing alveolar surface tension, and allowing for proper lung expansion during inhalation. Surfactant deficiency occurs when the surfactant production, secretion, or function is impaired, leading to increased surface tension and lung dysfunction.

**Why the Correct Answer is Right:**
Surfactant deficiency primarily leads to the development of respiratory distress syndrome (RDS), also known as hyaline membrane disease (HMD), in premature infants. The surfactant in premature infants is often immature and inadequate, causing increased surface tension in the lungs, leading to respiratory distress. This surfactant deficiency results in the formation of hyaline membranes (consisting of extracellular fibrin, fibronectin, and alveolar type II epithelial cells) and an inability to maintain lung function.

**Why Each Wrong Option is Incorrect:**

B. Pulmonary fibrosis is a condition characterized by the excessive accumulation of collagen and other extracellular matrix proteins, leading to lung scarring and impaired lung function. Surfactant deficiency does not directly cause pulmonary fibrosis.

C. Congenital hyaline membrane disease (HMD) is a severe respiratory distress syndrome primarily affecting premature infants due to immature lungs and inadequate surfactant production. It is not specifically caused by surfactant deficiency but by the immaturity of the lungs and the inadequate surfactant production in premature infants.

D. Pulmonary emphysema is a condition characterized by the destruction of lung tissue, leading to air-filled sacs (alveoli) enlargement and impaired gas exchange. Surfactant deficiency does not directly cause pulmonary emphysema; rather, it contributes to the development of respiratory distress syndrome (RDS) and hyaline membrane disease (HMD) in premature infants.

**Clinical Pearl:** Prophylactic administration of surfactant replacement therapy has been shown to improve the survival and outcome of premature infants with respiratory distress syndrome (RDS) and hyaline membrane disease (HMD). This therapy ameliorates the lung function impairment due to surfactant deficiency, thereby reducing the severity of RDS and HMD in premature infants.