**Core Concept**
The torus aorticus, also known as the aortic body, is a small group of chemoreceptors located in the aortic arch. These receptors play a crucial role in detecting changes in oxygen levels in the blood and triggering responses to maintain homeostasis.

**Why the Correct Answer is Right**
The torus aorticus is situated in the aortic arch, near the origin of the brachiocephalic artery. This location allows for the detection of oxygen levels in the blood as it flows from the heart to the rest of the body. The chemoreceptors in this region respond to decreases in oxygen tension, increases in carbon dioxide levels, and increases in hydrogen ion concentration, triggering a response to increase ventilation and perfusion.

**Why Each Wrong Option is Incorrect**
* **Option A:** The carotid body is a separate group of chemoreceptors located in the carotid bifurcation, not in the aortic arch.
* **Option B:** The aortic arch is the correct location, but the torus aorticus is not located in the descending aorta.
* **Option C:** The pulmonary arteries are not related to the location of the torus aorticus.

**Clinical Pearl / High-Yield Fact**
The torus aorticus is one of the three groups of chemoreceptors in the body, along with the carotid body and the peripheral arterial chemoreceptors. These receptors play a vital role in maintaining homeostasis and are often affected in conditions such as chronic obstructive pulmonary disease (COPD).

**Correct Answer:** D.