**Core Concept**
Egg is a type of dietary protein that contains essential amino acids necessary for growth and repair of body tissues. Amino acid deficiency can lead to various health problems, including impaired protein synthesis and muscle wasting.

**Why the Correct Answer is Right**
The correct answer is Vitamin D. Eggs are a significant source of several essential nutrients, but they are relatively deficient in Vitamin D. Vitamin D is crucial for maintaining bone health, immune function, and overall well-being. The lack of Vitamin D in eggs is due to the fact that chickens, the primary source of eggs, do not produce Vitamin D in their livers, which are the primary site of Vitamin D synthesis in animals.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we cannot evaluate its correctness.
**Option B:** This option is also not provided, so we cannot evaluate its correctness.
**Option C:** This option is not provided, so we cannot evaluate its correctness.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while eggs are relatively deficient in Vitamin D, they are an excellent source of other essential nutrients like protein, vitamin B12, and selenium. A balanced diet that includes a variety of food sources can help meet daily nutritional needs.

**Correct Answer: D. Vitamin D.