**Core Concept:** Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis, primarily affecting the lungs (primary pulmonary tuberculosis). The disease can present with various clinical manifestations, including cavitary lesions, pleural effusion, fibrocaseous lesions, and phlegmonous keratitis (not relevant to pulmonary TB).

**Why the Correct Answer is Right:**

The correct answer is B (pleural effusion) because pleural effusion is a common complication of primary pulmonary tuberculosis, particularly in its early stages. It occurs when the infection spreads to the pleural space, causing fluid buildup between the two pleural layers, leading to reduced lung expansion, breathlessness, and coughing.

**Why Each Wrong Option is Incorrect:**

A) Cavitary lesions are actually a characteristic feature of tuberculous pneumonia, not primary pulmonary tuberculosis. They represent lung destruction and cavitation caused by the inflammatory response to the infection.

C) Fibrocaseous lesions refer to the formation of caseous necrosis (a yellowish-white, necrotic tissue) within the cavities of tuberculosis. These lesions are more commonly seen in miliary tuberculosis or disseminated disease, not primary pulmonary tuberculosis.

D) Phlegmonous keratitis is a type of keratitis (inflammation of the cornea) caused by bacterial infections, including TB. However, it is not a typical manifestation of primary pulmonary tuberculosis and is more commonly associated with ocular tuberculosis.

**Clinical Pearl:** The key to recognizing primary pulmonary tuberculosis lies in understanding its clinical presentation, which may include fever, cough, and weight loss in addition to pleural effusion. Early diagnosis and treatment are crucial in preventing the progression of the disease to more severe forms, such as miliary tuberculosis or disseminated disease.

**Correct Answer:** B (pleural effusion)