**Core Concept**
Atherosclerosis is a chronic inflammatory disease of the arterial wall, characterized by the accumulation of lipids, inflammatory cells, and fibrous elements. The earliest lesion in atherosclerosis is a focal thickening of the intima, known as the fatty streak.

**Why the Correct Answer is Right**
The fatty streak is the earliest morphologically identifiable lesion in atherosclerosis. It is characterized by the accumulation of lipid-laden macrophages, known as foam cells, within the intima. The foam cells are formed when macrophages engulf lipids from the blood, leading to an increase in cell size and a loss of their ability to migrate. The accumulation of foam cells in the intima leads to an inflammatory response, which is a key component of the development of atherosclerosis.

**Why Each Wrong Option is Incorrect**
**Option A:** The fibrous plaque is a more advanced lesion in atherosclerosis, characterized by the accumulation of fibrous elements, such as collagen and smooth muscle cells, in addition to lipid-laden macrophages.
**Option B:** The calcified nodule is a feature of more advanced atherosclerosis, where calcium deposits accumulate within the plaque.
**Option C:** The thrombus is a blood clot that forms within a pre-existing plaque, leading to acute ischemic events, such as myocardial infarction.

**Clinical Pearl / High-Yield Fact**
The fatty streak is a precursor to more advanced lesions, such as the fibrous plaque and the calcified nodule. Understanding the progression of atherosclerotic lesions is crucial for the development of effective therapeutic strategies.

**Correct Answer: A. Fatty streak.**