**Core Concept**
In pulmonary tuberculosis, the initial site of infection is often the lung parenchyma, typically in the upper lobes, due to the higher concentration of alveolar macrophages and the subsequent deposition of inhaled Mycobacterium tuberculosis. The apical segment of the upper lobe is a common location for primary tuberculosis lesions, which can develop into Ghon's complex, a characteristic pathological finding.

**Why the Correct Answer is Right**
The correct answer is A. A single lesion in a lung apex because the apical segment of the upper lobe is a common site for the primary lesion of pulmonary tuberculosis. This is due to the higher concentration of alveolar macrophages in this region, which facilitates the initial deposition of M. tuberculosis. As a result, the primary lesion often develops in this area, leading to the characteristic Ghon's complex, which includes the primary lesion, associated lymphadenopathy, and, occasionally, pleural effusion.

**Why Each Wrong Option is Incorrect**
**Option B:** A single lesion in the gastrointestinal tract is incorrect because gastrointestinal tuberculosis is a rare manifestation of TB, usually occurring in individuals with a compromised immune system or those with a weakened gut barrier. It is not a common presentation of newly diagnosed pulmonary tuberculosis.
**Option C:** A single lesion subjacent to the pleura is incorrect because while pleural involvement can occur in pulmonary TB, a single lesion subjacent to the pleura is not a characteristic presentation of the disease.
**Option D:** Both a lesion subjacent to the pleura in the lower part of an upper lobe and active disease in the mediastinal lymph nodes is incorrect because, although pleural and lymph node involvement can occur in pulmonary TB, this option describes a more advanced or disseminated form of the disease, which is not typically seen in newly diagnosed cases.

**Clinical Pearl / High-Yield Fact**
In pulmonary tuberculosis, the apical segment of the upper lobe is often the site of the primary lesion due to the higher concentration of alveolar macrophages, which facilitates the initial deposition of M. tuberculosis. This knowledge is crucial for radiologists and clinicians to accurately diagnose and manage pulmonary TB.

**✓ Correct Answer: A. A single lesion in a lung apex**