**Core Concept**
Tuberculosis (T.B.) is a bacterial infection caused by Mycobacterium tuberculosis, which primarily affects the lungs but can spread to other parts of the body, including the kidneys. The kidneys are a common site of extrapulmonary T.B., and understanding the route of infection is crucial for diagnosis and treatment.

**Why the Correct Answer is Right**
The most common route of infection of the kidneys in T.B. is hematogenous, meaning the bacteria spread through the bloodstream from a primary site of infection, usually the lungs. This occurs when the bacteria are ingested by macrophages in the lungs, which then become infected and release the bacteria into the bloodstream. The bacteria are then carried by the bloodstream to the kidneys, where they can cause infection. The hematogenous route is favored by the high blood flow to the kidneys and the presence of macrophages in the renal interstitium.

**Why Each Wrong Option is Incorrect**
**Option A:** Direct invasion is not the primary route of infection for the kidneys in T.B. While direct invasion can occur, it is not the most common route.
**Option C:** Lymphatic spread is not a significant route of infection for the kidneys in T.B. Lymphatic spread is more common in other types of infections.
**Option D:** Ascending infection is characteristic of urinary tract infections, such as pyelonephritis, but is not the primary route of infection for the kidneys in T.B.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the kidneys are a common site of extrapulmonary T.B., and the hematogenous route of infection is favored by the high blood flow to the kidneys. This highlights the importance of considering T.B. in the differential diagnosis of renal infections, especially in patients with a history of pulmonary T.B. or exposure to someone with T.B.

**✓ Correct Answer: B. Hematogenous**