**Core Concept**
Keshan disease is a congenital cardiomyopathy characterized by severe heart damage and a high mortality rate in infants and children. This condition is caused by a deficiency of a specific micronutrient essential for the proper functioning of the heart.

**Why the Correct Answer is Right**
Keshan disease is caused by a deficiency of selenium, an essential trace element that plays a crucial role in the synthesis of glutathione peroxidase, an enzyme involved in the reduction of oxidative stress and lipid peroxidation in cardiac cells. Selenium supplementation has been shown to prevent the development of Keshan disease, highlighting its importance in maintaining cardiac health. The deficiency of selenium leads to the accumulation of malondialdehyde, a marker of lipid peroxidation, resulting in cardiac damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiamine (Vitamin B1) deficiency causes beriberi, a disease characterized by muscle weakness, nerve damage, and cardiovascular problems, but it is not associated with Keshan disease.

**Option B:** Zinc is an essential mineral that plays a critical role in immune function, wound healing, and protein synthesis, but its deficiency does not cause Keshan disease.

**Option D:** Protein deficiency can lead to kwashiorkor, a condition characterized by edema, muscle wasting, and impaired immune function, but it is not directly linked to Keshan disease.

**Clinical Pearl / High-Yield Fact**
Keshan disease is a classic example of a condition caused by a micronutrient deficiency, highlighting the importance of maintaining adequate levels of essential trace elements for optimal health.

**✓ Correct Answer: C. Selenium**