**Core Concept**
Vitamin A deficiency in children can be an indicator of malnutrition. Malnutrition in children is classified based on weight-for-age, which is a measure of a child's weight in relation to their age. The classification includes normal, underweight (or malnourished), and wasting.

**Why the Correct Answer is Right**
Malnutrition in children is graded based on the weight-for-age Z-score, which is calculated using the World Health Organization (WHO) growth charts. A weight-for-age Z-score less than -2 standard deviations indicates malnutrition. In this case, a child weighing 6.4 kg at 2 years of age is significantly underweight, indicating malnutrition.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.
* **Option D:** Without knowing the actual weight-for-age Z-score, this option cannot be evaluated.

**Clinical Pearl / High-Yield Fact**
In children, vitamin A deficiency can be a marker of malnutrition, and a weight-for-age Z-score less than -2 standard deviations indicates significant underweight, which is a form of malnutrition.

**Correct Answer: A.**