**Core Concept**
The acinus is a functional unit of the lung responsible for gas exchange and is divided into different segments. It consists of a cluster of alveoli surrounded by a network of capillaries and a duct that connects to a terminal bronchiole.

**Why the Correct Answer is Right**
The acinus is made up of terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveoli. However, the alveolar sacs are not a separate component of the acinus but rather a continuation of the alveolar ducts. The alveolar sacs are the final airspaces where gas exchange occurs and are not a distinct part of the acinus.

**Why Each Wrong Option is Incorrect**
**Option A:** Terminal bronchioles are indeed a component of the acinus, serving as the entrance to the acinar units.
**Option B:** Respiratory bronchioles are also a component of the acinus, where gas exchange begins to occur.
**Option D:** Alveolar ducts are a component of the acinus, leading to the alveolar sacs where gas exchange occurs.

**Clinical Pearl / High-Yield Fact**
Remember that the acinus is the functional unit of the lung where gas exchange occurs, and it is essential to understand its structure and components for diagnosing and managing respiratory diseases.

**Correct Answer: D. Alveolar sacs. Alveolar sacs are not a separate component of the acinus.**