**Core Concept**
Colostrum is the first milk produced by mammary glands during the initial 3-5 days postpartum, containing high levels of immunoglobulins, growth factors, and electrolytes. Its composition is distinct from regular milk, which is produced after lactogenesis II, around 3-5 days postpartum.

**Why the Correct Answer is Right**
Colostrum's high immunoglobulin content (primarily IgG) provides essential antibodies to newborns, protecting them from infections. Additionally, colostrum contains growth factors such as epidermal growth factor (EGF), which promote cellular growth and differentiation. The higher electrolyte concentration in colostrum helps in maintaining the newborn's electrolyte balance.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as colostrum does not have a lower immunoglobulin content compared to regular milk. In fact, it is the opposite.
**Option B:** This option is not accurate as colostrum is not primarily composed of water, but rather has a higher concentration of solids and electrolytes.
**Option C:** This option is incorrect as colostrum does not have a lower growth factor content compared to regular milk. In fact, colostrum contains higher levels of growth factors.

**Clinical Pearl / High-Yield Fact**
A crucial clinical fact to remember is that colostrum is an essential source of antibodies for newborns, and its consumption should be encouraged in the first few days of life to provide optimal protection against infections.

**Correct Answer: D.**