**Core Concept**
The question tests the student's knowledge of the pathophysiology of infectious diseases, specifically the clinical and histopathological features of tuberculosis (TB). TB is a chronic bacterial infection caused by Mycobacterium tuberculosis, characterized by granulomatous inflammation and lymphocytic infiltrates in the lungs.

**Why the Correct Answer is Right**
The correct answer is **Mycobacterium tuberculosis**. The presence of chronic cough, fever, and small, ill-defined nodular opacities in all lung fields on chest radiograph is highly suggestive of pulmonary TB. The transbronchial biopsy specimen showing interstitial infiltrates with lymphocytes, plasma cells, and epithelioid macrophages is consistent with the granulomatous inflammation characteristic of TB. The epithelioid macrophages are a hallmark of TB, and their presence in the biopsy specimen points towards this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** **Mycoplasma pneumoniae** is a cause of community-acquired pneumonia, but it typically presents with a more acute onset and is less likely to cause chronic cough and granulomatous inflammation.

**Option B:** **Histoplasma capsulatum** is a fungal pathogen that can cause histoplasmosis, but it typically presents with a more acute onset and is less likely to cause chronic cough and granulomatous inflammation.

**Option C:** **Pneumocystis jirovecii** is a fungal pathogen that can cause pneumocystis pneumonia, but it typically presents in immunocompromised patients and is less likely to cause chronic cough and granulomatous inflammation.

**Clinical Pearl / High-Yield Fact**
In patients with chronic cough and fever, a chest radiograph showing small, ill-defined nodular opacities in all lung fields should raise suspicion for tuberculosis. A transbronchial biopsy specimen showing interstitial infiltrates with lymphocytes, plasma cells, and epithelioid macrophages is highly suggestive of TB.

**Correct Answer:** C. **Mycobacterium tuberculosis**