**Core Concept**
Premature milk, also known as transitional milk, is produced by the mother's body in response to the stress of premature birth. This type of milk has a different composition than mature milk, which is produced after several weeks of lactation.

**Why the Correct Answer is Right**
Lactose, a disaccharide composed of glucose and galactose, is a crucial energy source for the developing infant. However, premature milk contains lower levels of lactose due to the mother's body adapting to the stress of premature delivery. This reduction in lactose is compensated by an increase in lipids and proteins to meet the infant's energy needs. The decrease in lactose is mediated by the enzyme lactase, which breaks down lactose into glucose and galactose.

**Why Each Wrong Option is Incorrect**
* **Option B:** Fat content in premature milk is actually higher than in mature milk, as it provides essential energy for the infant's growth and development.
* **Option C:** Protein content in premature milk is also higher than in mature milk, as it supports the infant's rapid growth and development during this critical period.
* **Option D:** Sodium content in premature milk is actually lower than in mature milk, as the mother's body tries to conserve sodium and other electrolytes in response to the stress of premature delivery.

**Clinical Pearl / High-Yield Fact**
It is essential to note that premature milk is rich in immunoglobulins, which provide passive immunity to the infant. This makes premature milk a vital source of protection against infections in preterm infants.

**✓ Correct Answer: A. Lactose**