**Core Concept**
Breast milk is a complete and balanced diet for infants, providing all necessary nutrients for growth and development. However, breast milk is deficient in certain nutrients, particularly water-soluble vitamins, which are essential for various bodily functions.

**Why the Correct Answer is Right**
Vitamin B12 (cobalamin) is a crucial water-soluble vitamin that is present in small amounts in breast milk. Since breast milk is the sole source of nutrition for exclusively breastfed babies, they are at risk of developing vitamin B12 deficiency, particularly after 6 months of age. Vitamin B12 plays a vital role in the synthesis of DNA, fatty acid metabolism, and the maintenance of the nervous system. Infants with vitamin B12 deficiency may exhibit symptoms such as anemia, neurological problems, and impaired cognitive development.

**Why Each Wrong Option is Incorrect**
**Option B:** Vitamin A is actually present in breast milk and is essential for vision, immune function, and skin health. Breastfed infants may have a higher risk of vitamin A deficiency if their mothers have inadequate dietary intake or liver stores.

**Option C:** Vitamin C is present in breast milk, although in smaller amounts than in formula. Breastfed infants typically have adequate vitamin C levels, and deficiency is rare.

**Option D:** Proteins are abundant in breast milk, providing essential amino acids for growth and development. Breast milk contains a unique balance of proteins, including lactoferrin and lysozyme, which support infant health.

**Clinical Pearl / High-Yield Fact**
Exclusively breastfed infants are at risk of vitamin B12 deficiency after 6 months of age. This highlights the importance of monitoring for signs of deficiency, such as anemia, and ensuring adequate maternal vitamin B12 status through dietary supplements or fortified foods.

**✓ Correct Answer: A. Vitamin B**