**Core Concept**
Kidney tuberculosis (TB) is a form of extrapulmonary tuberculosis that affects the kidneys. The disease is caused by the bacterium *Mycobacterium tuberculosis*. Infection occurs when the bacteria are ingested or inhaled and then spread to other parts of the body through the bloodstream or lymphatic system.

**Why the Correct Answer is Right**
The most common route of infection in kidney TB is hematogenous spread from a primary focus, typically the lungs. This occurs when the bacteria are ingested or inhaled and then multiply in the lungs, causing a primary infection. From there, they can spread to other parts of the body through the bloodstream, including the kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** Direct extension from a contiguous focus is a possible route of infection, but it is less common than hematogenous spread.
**Option B:** Urinary tract infection is not a primary route of infection in kidney TB, as the bacteria are not typically acquired through urinary tract exposure.
**Option C:** Fecal-oral transmission is a route of infection for intestinal TB, but it is not relevant to kidney TB.

**Clinical Pearl / High-Yield Fact**
Remember that kidney TB often presents with non-specific symptoms, such as flank pain, hematuria, and sterile pyuria. A high index of suspicion is necessary for diagnosis, as the clinical presentation can be misleading.

**Correct Answer: A. Hematogenous spread.**