**Core Concept**
Human milk is a dynamic substance influenced by the mother's health and the baby's needs, but certain conditions can make breastfeeding contraindicated. The decision to breastfeed is a complex one, weighing the benefits against potential risks, particularly for the baby.

**Why the Correct Answer is Right**
In cases of active tuberculosis (TB), breastfeeding is contraindicated due to the risk of transmitting the Mycobacterium tuberculosis bacterium to the infant. This is especially concerning in areas where TB is prevalent and in cases where the mother has a high infectious load. The baby is at risk of developing TB, which can manifest as miliary TB, a severe form of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** HIV infection is not a contraindication for breastfeeding, as the benefits of breast milk for the baby, such as immune system development, outweigh the risks. The mother can be on antiretroviral therapy (ART) to reduce the risk of HIV transmission.
**Option B:** Mastitis, an inflammation of the breast tissue, is not a contraindication for breastfeeding. In fact, ongoing breastfeeding can help clear the infection.
**Option C:** Galactosemia, a genetic disorder affecting glucose metabolism, is not a contraindication for breastfeeding. Women with galactosemia can breastfeed, but the baby may need to follow a lactose-free diet.

**Clinical Pearl / High-Yield Fact**
In areas where TB is prevalent, a thorough TB screening should be performed before initiating breastfeeding, and the mother should be on effective TB treatment if infected.

**Correct Answer: D. Active tuberculosis**