**Core Concept**
Breast milk is a dynamic and nutritious fluid providing essential nutrients for infants. Its composition varies over time, reflecting the mother's diet, age, and lactation stage. However, certain nutrients are either present in lower concentrations or entirely absent, posing potential risks for infant development.

**Why the Correct Answer is Right**
Breast milk is naturally deficient in vitamin B12, a crucial nutrient for the synthesis of DNA, fatty acid metabolism, and the maintenance of the nervous system. This deficiency arises due to the limited production of vitamin B12 in the human gut, which is then supplemented by dietary intake. Since breast milk does not contain significant amounts of vitamin B12, infants rely on maternal stores and, eventually, external sources for adequate supplies.

**Why Each Wrong Option is Incorrect**
**Option A:** While breast milk may contain lower levels of certain nutrients, it is not typically deficient in iron. In fact, human milk contains sufficient iron for infants during the first six months of life.
**Option B:** Breast milk is not deficient in calcium, which is essential for bone growth and development. Human milk contains adequate amounts of calcium to support infant bone health.
**Option C:** Breast milk does not contain cholesterol, but it is not the correct answer in this context. Human milk contains lipids, including essential fatty acids, but the composition and quantity vary over time.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare providers to monitor and address potential vitamin B12 deficiencies in infants, particularly those born to vegan or vegetarian mothers or those with a family history of vitamin B12 deficiency.

**Correct Answer: C. Cholesterol**