**Core Concept:**
Pulmonary surfactant is a mixture of lipids and proteins found in the alveoli of the lungs, which reduces surface tension at the air-liquid interface, allowing for efficient gas exchange and preventing alveolar collapse. It plays a crucial role in maintaining lung function.

**Why the Correct Answer is Right:**
The correct answer is **D:** Pulmonary surfactant is synthesized by type II pneumocytes, which are specialized alveolar cells responsible for producing this essential substance. Type II pneumocytes secrete surfactant into the alveoli, ensuring proper functioning of the lungs.

**Why Each Wrong Option is Incorrect:**
A) **Option A:** Epithelial cells (type I pneumocytes) are responsible for the maintenance of the alveolar structure and fluid balance, not surfactant synthesis.
B) **Option B:** Macrophages are immune cells involved in phagocytosis and do not synthesize surfactant.
C) **Option C:** Capillary endothelial cells are part of the blood vessel lining of the alveoli but do not produce surfactant.

**Clinical Pearl / High-Yield Fact:**
Understanding the anatomy and physiology of pulmonary surfactant synthesis can help you differentiate between various lung diseases, such as those affecting surfactant production (e.g., respiratory distress syndrome in premature infants) or those causing structural abnormalities where surfactant production remains normal (e.g., interstitial lung diseases).

**Correct Answer:** D) Pulmonary surfactant is synthesized by type II pneumocytes.