**Core Concept**
Reactivation tuberculosis (TB) is a type of latent TB infection that reactivates in an individual, typically due to a decline in cellular immunity. This reactivation is almost exclusively associated with the lungs, as they are the primary site of TB infection in the majority of cases.

**Why the Correct Answer is Right**
The lungs are the primary site of TB infection due to the presence of alveolar macrophages, which are responsible for the initial phagocytosis of Mycobacterium tuberculosis (M. tuberculosis) bacteria. However, if the individual's cellular immunity is compromised, the bacteria can reactivate, leading to the formation of tubercles and caseation necrosis in the lung tissue. This is why reactivation TB is almost exclusively a disease of the lungs.

**Why Each Wrong Option is Incorrect**
**Option B:** Bones can be affected in cases of extrapulmonary TB, particularly in individuals with compromised immunity, but they are not the primary site of reactivation TB.
**Option C:** Joints can be involved in cases of TB, such as TB arthritis, but they are not the primary site of reactivation TB.
**Option D:** The brain can be affected in cases of TB meningitis, but it is not the primary site of reactivation TB.

**Clinical Pearl / High-Yield Fact**
It's essential to note that reactivation TB can occur in any individual with latent TB infection, but it is more common in individuals with compromised cellular immunity, such as those with HIV/AIDS, chronic kidney disease, or taking immunosuppressive therapy.

**✓ Correct Answer: A. Lungs**