**Core Concept**
Arterial thromboembolism refers to the blockage of an artery caused by a blood clot (thrombus) that breaks loose and travels to another part of the body, known as an embolus. This can lead to ischemia and infarction in various organs, depending on the location and size of the embolus.

**Why the Correct Answer is Right**
The brain is often considered the least affected organ in arterial thromboembolism due to its dual blood supply, which includes the internal carotid and vertebral arteries. This redundancy allows some brain tissue to remain perfused even if one of these arteries is occluded by an embolus. Additionally, the brain's ability to adapt and compensate for reduced blood flow through collateral circulation also contributes to its relative resistance to ischemic damage.

**Why Each Wrong Option is Incorrect**
**Option A:** The heart is highly susceptible to damage from arterial thromboembolism, particularly in the context of acute myocardial infarction (heart attack). The coronary arteries supply the heart muscle itself, and occlusion of these arteries can lead to rapid and severe damage.
**Option B:** The kidneys are also vulnerable to ischemic damage from arterial thromboembolism, particularly in the context of acute kidney injury (AKI). The renal arteries supply the kidneys, and occlusion of these arteries can lead to rapid loss of renal function.
**Option C:** The spleen is a highly vascular organ that is also susceptible to damage from arterial thromboembolism. The splenic artery supplies the spleen, and occlusion of this artery can lead to splenic infarction.

**Clinical Pearl / High-Yield Fact**
The "watershed" areas of the brain, which are supplied by the terminal branches of the middle and anterior cerebral arteries, are particularly susceptible to ischemic damage due to their limited collateral blood supply.

**Correct Answer: C. Spleen. Spleen**