**Core Concept**
Tuberculosis (T.B.) testis affects the testes due to hematogenous spread from a primary focus, typically the lungs. The primary focus of T.B. is usually the lung, where the bacteria Mycobacterium tuberculosis infects the alveoli and then spreads to other parts of the body through the bloodstream.

**Why the Correct Answer is Right**
The correct answer is the epididymis, as it is the most common site of T.B. testis due to its rich blood supply and the predilection of the bacteria to infect areas with a high concentration of macrophages. The epididymis is a long, coiled tube that stores and transports sperm from the testes, and its rich blood supply makes it an ideal site for the bacteria to establish a foothold. The bacteria then spread from the epididymis to the testes, causing inflammation and damage to the testicular tissue.

**Why Each Wrong Option is Incorrect**
* **Option A:** The testes are not the primary site of T.B. testis. While the testes can be affected, the epididymis is the most common site of infection.
* **Option B:** The vas deferens is a muscular tube that transports sperm from the epididymis to the prostate gland, but it is not the first site affected by T.B. testis.
* **Option C:** The prostate gland is a common site of T.B. in men, but it is not the first site affected in T.B. testis.

**Clinical Pearl / High-Yield Fact**
The epididymis is the most common site of T.B. testis due to its rich blood supply and the predilection of the bacteria to infect areas with a high concentration of macrophages. This is a classic exam trap, as many students assume that the testes are the primary site of T.B. testis.

**Correct Answer:** C. The epididymis.