**Core Concept**
Keshan disease is a myocardial disorder caused by a deficiency of a specific micronutrient essential for maintaining cardiac function and overall cardiovascular health. It primarily affects children and young adults, leading to cardiomyopathy and potentially life-threatening complications.

**Why the Correct Answer is Right**
Keshan disease is caused by a deficiency of selenium (Se), an essential trace element that acts as a cofactor for several enzymes involved in antioxidant defenses and thyroid hormone metabolism. Selenium deficiency impairs the activity of these enzymes, leading to the accumulation of reactive oxygen species and subsequent damage to cardiac tissue. This results in the characteristic features of Keshan disease, including enlarged heart, congestive heart failure, and arrhythmias.

**Why Each Wrong Option is Incorrect**
**Option A:** Zinc deficiency is associated with growth retardation, impaired immune function, and increased susceptibility to infections, but it is not directly linked to Keshan disease.
**Option B:** Iodine deficiency is a well-known cause of goiter and hypothyroidism, but it is not related to the pathogenesis of Keshan disease.
**Option C:** Copper deficiency can lead to anemia, neutropenia, and impaired immune function, but it is not a known cause of Keshan disease.

**Clinical Pearl / High-Yield Fact**
Keshan disease is a classic example of a selenium deficiency disorder, highlighting the importance of adequate selenium intake in maintaining cardiovascular health, particularly in populations with limited access to selenium-rich food sources.

**Correct Answer: D. Selenium. Selenium deficiency.**