**Core Concept**
Cardiac myxoma is a type of benign tumor that arises from the cardiac interstitium, often located in the left atrium. This tumor type is characterized by the accumulation of a specific substance, which is a key distinguishing feature.

**Why the Correct Answer is Right**
The substance that accumulates in cardiac myxoma is mucin, a type of glycoprotein. Mucin production is a result of the abnormal proliferation of interstitial cells, which are the cells from which cardiac myxomas arise. These cells produce and secrete mucin, leading to the accumulation of this substance within the tumor. This mucin deposition is a hallmark of cardiac myxoma and can be used as a diagnostic criterion.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the accumulation of substance in cardiac myxoma.
* **Option B:** This option is incorrect because it is not the substance that accumulates in cardiac myxoma.
* **Option D:** This option is incorrect because it is not the correct substance that accumulates in cardiac myxoma.

**Clinical Pearl / High-Yield Fact**
Cardiac myxomas are often associated with embolic phenomena due to the production of mucin, which can lead to the formation of thrombi and subsequent embolization.

**Correct Answer: C. Mucin**