**Core Concept**
Primary tuberculosis (TB) is an initial infection with Mycobacterium tuberculosis, primarily affecting the lungs. It is characterized by the formation of a primary lesion, typically in the lower lobe of the lung, due to the inhalation of the bacteria.

**Why the Correct Answer is Right**
The correct answer is the middle lobe of the right lung. This is because the right middle lobe is the most common site for primary TB lesions due to the anatomy of the right lung. The right middle lobe is more prone to infections because of its relatively smaller size and the presence of the right major bronchus, which is more vertical and has a greater tendency to become obstructed by secretions and debris. This obstruction leads to stasis of air and an increased risk of infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Upper lobe of the left lung - While the upper lobes are commonly affected in secondary TB, they are less commonly involved in primary TB.
* **Option B:** Lower lobe of the left lung - Although the lower lobes are the most common site for primary TB, the lower lobe of the left lung is less commonly affected than the middle lobe of the right lung.
* **Option D:** Upper lobe of the right lung - The upper lobes are less commonly involved in primary TB, and the middle lobe of the right lung is the most common site.

**Clinical Pearl / High-Yield Fact**
In primary TB, the primary lesion often calcifies and becomes a Ghon complex, which can be seen on chest X-rays as a calcified nodule. This calcification occurs in up to 90% of cases and is a hallmark of primary TB.

**Correct Answer: C. Middle lobe of the right lung.**