**Core Concept**
Vitamin D is an essential fat-soluble vitamin that plays a crucial role in maintaining bone health, calcium homeostasis, and immune function. The daily requirement of Vitamin D varies across different age groups, including infants and children.

**Why the Correct Answer is Right**
The recommended daily intake of Vitamin D for infants and children is crucial for optimal growth and development. The American Academy of Pediatrics recommends that infants and children receive 400 IU (International Units) of Vitamin D per day to maintain adequate levels of 25-hydroxyvitamin D, the primary circulating form of Vitamin D. This is particularly important during the first two years of life when bone growth and development are most rapid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the recommended daily intake of Vitamin D for infants and children is not 200 IU. While this may be sufficient for some individuals, it falls short of the recommended amount for optimal growth and development.
**Option B:** This option is incorrect because the recommended daily intake of Vitamin D for infants and children is not 600 IU. While this may be sufficient for some individuals, it exceeds the recommended amount for most children.
**Option C:** This option is incorrect because the recommended daily intake of Vitamin D for infants and children is not 100 IU. This amount is too low to meet the needs of most children, particularly during periods of rapid growth and development.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Vitamin D deficiency can lead to rickets in children, a condition characterized by softening of the bones, which can result in bowed legs, stunted growth, and increased risk of fractures.

**Correct Answer:** D. 400 IU