**Core Concept**
Keshan's disease is a cardiomyopathy characterized by dilated cardiomyopathy and congestive heart failure, primarily affecting children and young women. It is a classic example of a nutritional deficiency disorder with a specific biochemical basis.

**Why the Correct Answer is Right**
Keshan's 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 lipid peroxides and protection against oxidative stress. Selenium is also involved in the functioning of other enzymes, such as thioredoxin reductase, which helps to maintain the balance between oxidation and reduction reactions in the body. The deficiency of selenium leads to an imbalance in the antioxidant defense system, resulting in the accumulation of lipid peroxides and damage to cardiac tissue.

**Why Each Wrong Option is Incorrect**
**Option B:** Zinc is an essential trace element involved in numerous biochemical reactions, including protein synthesis, wound healing, and immune function. However, its deficiency is associated with conditions such as acrodermatitis enteropathica, growth retardation, and impaired immune function, but not Keshan's disease.
**Option C:** Copper is another essential trace element involved in the synthesis of hemoglobin, melanin, and connective tissue. Its deficiency can lead to conditions such as anemia, neutropenia, and osteoporosis, but it is not directly related to Keshan's disease.
**Option D:** Iron is a critical element involved in the synthesis of hemoglobin, myoglobin, and certain enzymes. Its deficiency can lead to conditions such as anemia, fatigue, and impaired cognitive function, but it is not associated with Keshan's disease.

**Clinical Pearl / High-Yield Fact**
Selenium deficiency can also lead to a condition known as Keshan's disease in rats, which is characterized by cardiomyopathy and congestive heart failure. This condition was first identified in the Keshan County of Heilongjiang Province in China, hence the name "Keshan's disease".

**✓ Correct Answer: A. Selenium. Selenium deficiency is the primary cause of Keshan's disease.**