**Core Concept**
The question is testing the understanding of the epidemiology of tuberculosis (TB), specifically the route of transmission in a high-risk individual. **Mycobacterium tuberculosis** is the causative agent of TB, and infection is typically acquired through inhalation of aerosolized droplets containing the bacteria.

**Why the Correct Answer is Right**
In individuals with compromised immune systems, such as those with HIV/AIDS or chronic drug abuse, the risk of developing active TB is significantly increased. The most common route of transmission is through inhalation of aerosolized droplets containing **M. tuberculosis**, often from an infected individual with active pulmonary TB. This is known as primary TB infection, which can progress to active TB disease if left untreated.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cutaneous TB is a rare form of TB that can be acquired through inoculation of bacteria into the skin, often through a contaminated needle or surgical instrument. This is not the most common route of transmission in a high-risk individual.
* **Option B:** Gastrointestinal TB is a form of extrapulmonary TB that can be acquired through ingestion of contaminated food or water. While possible, this is not the most common route of transmission in a high-risk individual.
* **Option D:** Hematogenous spread of TB is a rare form of TB that occurs through the bloodstream. While possible, this is not the most common route of transmission in a high-risk individual.

**Clinical Pearl / High-Yield Fact**
TB infection is often asymptomatic in the early stages, and the PPD test detects the presence of **M. tuberculosis** antibodies in the blood. However, a positive PPD test does not necessarily indicate active TB disease; it only indicates exposure to the bacteria.

**Correct Answer:** C.