**Core Concept**
Mother's milk is optimized for the nutritional needs of human infants, with a unique composition that differs significantly from cow's milk. Lactose, a disaccharide sugar, is a crucial component of human milk that serves as a primary source of energy for the growing infant.

**Why the Correct Answer is Right**
Human milk contains a higher concentration of lactose compared to cow's milk, which is essential for the infant's growth and development. The higher lactose content in mother's milk allows for easier digestion and absorption, particularly in the first few months of life. The enzyme lactase, present in the infant's small intestine, breaks down lactose into glucose and galactose, providing a readily available source of energy for the infant's brain, nervous system, and other vital organs.

**Why Each Wrong Option is Incorrect**
**Option B:** Vitamin D is actually present in lower concentrations in human milk compared to cow's milk, which is fortified with this essential nutrient to promote bone growth and development in infants.
**Option C:** Human milk contains a lower concentration of proteins compared to cow's milk, although the protein quality in human milk is superior due to the presence of casein and whey proteins that are easily digested by infants.
**Option D:** The fat content in human milk is also lower compared to cow's milk, although the fatty acid profile in human milk is more suitable for infant development and is easily digested.

**Clinical Pearl / High-Yield Fact**
It's essential to note that human milk is not simply a source of nutrition, but also provides essential immune factors, growth factors, and hormones that promote the development and health of the infant. This unique composition of human milk highlights the importance of breastfeeding as the optimal nutrition for infants.

**✓ Correct Answer: A. Lactose**