**Core Concept:**
Respiratory distress syndrome (RDS) is a condition that occurs in premature infants due to immaturity of the lungs, which leads to inadequate surfactant production. Surfactant is a mixture of phospholipids, proteins, and lipids that prevents alveoli from collapsing, allowing for normal breathing.

**Why the Correct Answer is Right:**
Respiratory distress syndrome is primarily caused by insufficient secretion of lung surfactant, specifically phosphatidylcholine (PC), which is a key component of surfactant. The premature infants lack the necessary enzymes involved in phosphatidylcholine synthesis, like lecithin synthase, resulting in low surfactant levels and increased alveolar surface tension.

**Why Each Wrong Option is Incorrect:**
A. Lecithin: This is another phospholipid component of surfactant, not the cause of RDS but a component that helps maintain lung function.
B. Phosphatidylethanolamine: Another phospholipid component of surfactant, but not directly related to surfactant deficiency in RDS.
C. Phosphatidylglycerol: A surfactant component, but not the primary cause of RDS.
D. Lecithin synthase: The enzyme responsible for phosphatidylcholine synthesis, not the cause of RDS but a key factor in surfactant production.

**Clinical Pearl:**
Understanding the role of surfactant in maintaining lung function and the specific deficiency in phosphatidylcholine is crucial for diagnosing and treating RDS in premature infants promptly. Early surfactant replacement therapy can improve survival and lung function in these patients.

**Correct Answer:**
D. Lecithin synthase (Phosphatidylcholine synthesis enzyme) is the correct answer because it is involved in the production of phosphatidylcholine, a crucial component of lung surfactant, and its deficiency in premature infants leads to respiratory distress syndrome.