**Core Concept**
Atheroma, also known as atherosclerosis, is a condition characterized by the buildup of plaque in the arterial walls, leading to angina in its early stages. The process involves the accumulation of lipids, macrophages, smooth muscle cells, and fibrous elements, resulting in a complex lesion.

**Why the Correct Answer is Right**
Atheromas typically exhibit a characteristic "foam cell" appearance due to the presence of macrophages that have ingested lipids. The macrophages play a crucial role in the development and progression of atherosclerosis by contributing to the inflammatory response and the formation of foam cells. In contrast, a lack of macrophage involvement is not a characteristic feature of atheromas.

**Why Each Wrong Option is Incorrect**
**Option A:** A thin fibrous cap is actually a characteristic feature of vulnerable atheromas, which are prone to rupture and acute coronary syndromes. This is in contrast to stable atheromas, which have a thicker fibrous cap.
**Option B:** A thick fibrous cap is a characteristic feature of stable atheromas, which are less likely to rupture and cause acute coronary syndromes. This is in contrast to vulnerable atheromas, which have a thin fibrous cap.
**Option D:** Smooth muscle cells are indeed involved in the development of atheromas, particularly in the formation of the fibrous cap. Their absence would not be a characteristic feature of atheromas.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the presence of macrophages in atheromas is a key indicator of inflammation and potential plaque instability. This knowledge can help clinicians identify patients at higher risk of acute coronary syndromes.

**✓ Correct Answer: C. Lack of macrophage**