**Core Concept:** Alveoli are the functional units of the respiratory system, where gas exchange occurs. They are connected to the bronchioles and are surrounded by a network of capillaries. The stability of alveoli is crucial for efficient gas exchange.

**Why the Correct Answer is Right:** The correct answer, D, refers to the surfactant, which is a complex mixture of phospholipids, proteins, and glycolipids secreted primarily by type II alveolar cells. Surfactant reduces surface tension within the alveoli, preventing them from collapsing. This is important because collapsed alveoli would reduce the surface area available for gas exchange, impairing oxygenation and carbon dioxide elimination.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Elastic recoil pressure - This refers to the intrinsic tension of the alveolar walls, which is important for maintaining alveolar shape but does not directly contribute to preventing collapse.

B. **Option B:** Alveolar walls - While the walls of the alveoli provide structural support, they do not directly prevent collapse. Surfactant plays the primary role in this regard.

C. **Option C:** Alveolar walls - Similar to option B, the alveolar walls contribute to structural support but do not directly prevent collapse.

**Clinical Pearl:** Surfactant deficiency or dysfunction can lead to respiratory distress syndrome, a life-threatening condition in newborns, known as hyaline membrane disease. This is a prime example of how a deficiency in surfactant can cause severe respiratory impairment.

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**Question:** Stability of alveoli is maintained by:

A. Elastic recoil pressure
B. Alveolar walls
C. Surfactant
D. None of the above

**Correct Answer:** C. Surfactant

**Core Concept:** The primary function of the alveoli is gas exchange, which depends on their shape and surface area. Surfactant plays a crucial role in maintaining the stability and functionality of alveoli by reducing surface tension within them.

**Why Each Wrong Option is Incorrect:**

A. Elastic recoil pressure is important for maintaining the shape of the alveoli but does not directly prevent collapse.
B. Alveolar walls provide structural support but do not directly prevent alveolar collapse.
D. None of the above options (incorrect) - Surfactant is the primary factor ensuring alveolar stability and gas exchange efficiency.

**Clinical Pearl:** Surfactant is crucial in neonatal respiratory care, particularly in premature infants. Premature birth often leads to surfactant deficiency, causing respiratory distress syndrome (RDS) and making the alveoli prone to collapsing. Replacing or supporting surfactant production in premature infants can improve respiratory function and reduce mortality and morbidity in this patient population.