Which of the following is the main component of surfactant, secreted by alveolar epithelial cells?
**Core Concept**
Surfactant is a complex mixture of lipids and proteins secreted by alveolar epithelial cells, playing a crucial role in reducing surface tension within the alveoli, thereby facilitating lung expansion and gas exchange. The main component of surfactant is a phospholipid that helps to stabilize the alveolar surface.
**Why the Correct Answer is Right**
The main component of surfactant is **phosphatidylcholine** (PC), also known as dipalmitoylphosphatidylcholine (DPPC). This phospholipid is synthesized by alveolar epithelial cells and secreted into the alveolar space, where it forms a monolayer that reduces surface tension. The hydrophobic tails of PC molecules face inward, towards the alveolar surface, while the hydrophilic heads face outward, towards the aqueous environment. This arrangement allows PC to effectively reduce surface tension and prevent alveolar collapse during expiration.
**Why Each Wrong Option is Incorrect**
* **Option B:** Surfactant protein A (SP-A) is a component of surfactant, but it is not the main lipid component. SP-A plays a crucial role in regulating surfactant function and immune defense.
* **Option C:** Phosphatidylethanolamine (PE) is another phospholipid found in surfactant, but it is not the main component. PE is involved in modulating surfactant function and stability.
* **Option D:** Cholesterol is a component of surfactant, but it is not the main phospholipid. Cholesterol helps to regulate surfactant function and stability, but it is not the primary contributor to surface tension reduction.
**Clinical Pearl / High-Yield Fact**
Surfactant deficiency or dysfunction can lead to respiratory distress syndrome (RDS) in preterm infants, characterized by difficulty breathing, rapid breathing rate, and decreased lung compliance. Administering exogenous surfactant can help to improve lung function and reduce mortality in these patients.
**Correct Answer:** C. Phosphatidylcholine.