**Core Concept**
Breast milk provides all the necessary nutrients for an infant's growth and development. However, it is often deficient in certain essential nutrients, particularly in the first few months of exclusive breastfeeding.

**Why the Correct Answer is Right**
The correct answer is Vitamin K. Breast milk contains low levels of Vitamin K, which is essential for blood clotting and preventing bleeding disorders in newborns. Vitamin K deficiency in infants can lead to bleeding complications, including intracranial hemorrhage. This is why Vitamin K injections are recommended for all newborns, regardless of whether they are breastfed or formula-fed.

**Why Each Wrong Option is Incorrect**
**Option A:** Folate - While breast milk is a good source of folate, it is not the primary concern in exclusively breastfed babies.

**Option B:** Vitamin B12 - Breast milk contains adequate amounts of Vitamin B12, and deficiency is rare in exclusively breastfed infants unless the mother is deficient.

**Option C:** Iron - Breast milk contains adequate iron for the first 4-6 months of life, and iron deficiency is rare in exclusively breastfed infants unless the mother is deficient.

**Clinical Pearl / High-Yield Fact**
Vitamin K deficiency bleeding (VKDB) is a significant concern in exclusively breastfed infants, particularly in the first few weeks of life. This highlights the importance of Vitamin K injections at birth, regardless of feeding method.

**Correct Answer:** D. Vitamin K