Which of the following amino acids is absent in human milk?
**Core Concept**
Human milk is a complex mixture of proteins, fats, carbohydrates, and other nutrients that provide essential nourishment for infants. The amino acid composition of human milk is carefully regulated to support growth and development.
**Why the Correct Answer is Right**
Human milk contains a range of essential and non-essential amino acids, but it lacks the amino acid **Tryptophan**. This is because human infants have a limited ability to synthesize tryptophan from other amino acids, and breast milk provides a sufficient amount of other amino acids to meet their needs. The absence of tryptophan in human milk is thought to be an adaptation to prevent the accumulation of kynurenine, a toxic metabolite of tryptophan.
**Why Each Wrong Option is Incorrect**
**Option A:** Taurine is actually present in human milk, where it serves as an antioxidant and helps to protect the infant's retina and heart.
**Option B:** Phenylalanine is an essential amino acid that is present in human milk, and infants require it for growth and development.
**Option C:** Histidine is another essential amino acid that is present in human milk, and it plays a critical role in the synthesis of histamine and other biologically active compounds.
**Clinical Pearl / High-Yield Fact**
Infants fed human milk have a lower risk of necrotizing enterocolitis (NEC) and other gastrointestinal disorders compared to formula-fed infants, which may be due in part to the unique amino acid composition of human milk.
**Correct Answer:** D. Tryptophan