Question

Respiratory distress syndrome is caused due to the absence of which lung surfactant of premature infants?

• A. Phosphatidic acid
• B. Dipalmitoyl phosphatidylcholine
• C. Dipalmitoyl serine
• D. Phosphatidylethanolamine

Hint:

The lecithin-to-sphingomyelin (L/S) ratio in amniotic fluid is a key prenatal diagnostic indicator of fetal lung development.
An L/S ratio greater than 0 indicates mature lungs.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Neonatal Respiratory Distress Syndrome (RDS) is a clinical condition that primarily affects premature infants.
It is caused by the developmental insufficiency of pulmonary surfactant production in the lungs.
Surfactant is a complex lipid-protein mixture that coats the inner surface of the alveoli, lowering surface tension and preventing collapse during expiration.

Step 2: Detailed Explanation:
Pulmonary surfactant is synthesized and stored in lamellar bodies by Type II alveolar epithelial cells.
Chemically, it consists of roughly $90\%$ lipids and $10\%$ proteins.
The major, physiologically active lipid component responsible for reducing alveolar surface tension is Dipalmitoyl phosphatidylcholine (DPPC), also commonly known as dipalmitoyl lecithin.
DPPC represents more than $50\%$ of the total lipid content in surfactant.
DPPC is a unique phospholipid because both of its fatty acid chains are saturated palmitic acids ($16:0$).
The absence of double bonds allows the hydrophobic tails of DPPC to pack tightly together when the surface area of the alveoli decreases during expiration.
This packing creates a highly cohesive intermolecular layer at the air-water interface, reducing the surface tension of the alveolar fluid to nearly zero.
Without adequate DPPC, the surface tension remains high, causing alveoli to collapse at the end of each breath (atelectasis).
This leads to increased work of breathing, hypoxia, and respiratory failure in premature infants whose lungs have not yet begun producing sufficient DPPC.

Step 3: Final Answer:
The missing lung surfactant component causing RDS is Dipalmitoyl phosphatidylcholine.