Which of the following substances is part of the combination found in Natural Moisturizing Factors?

Natural Moisturizing Factors (NMF) are a complex of water-soluble compounds that play a critical role in keeping the outer layer of the skin (the stratum corneum) hydrated. They help retain moisture within the skin, contributing to a healthy, plump appearance.

Amino acids are a vital component of NMF. They are derived from the breakdown of proteins and help to bind moisture to the skin. Their presence helps to maintain water content, which is essential for skin barrier function and overall skin health. By providing a reservoir of humectants, amino acids enhance the skin's ability to retain moisture, making them a key part of NMF.

Other substances mentioned, while beneficial for skin health, do not constitute part of NMF. For instance, collagen is primarily a structural protein that provides elasticity and strength to the skin, while hyaluronic acid is also a powerful humectant but is not classified as one of the primary components of NMF itself. Retinol, known for its role in skin cell turnover and anti-aging properties, does not play a direct role in the moisturizing factors of the skin. Thus, amino acids stand out as the correct and significant component of Natural Moisturizing Factors.