**Core Concept**
Pleural effusion is categorized based on its protein content, specific gravity, and cell type. A high specific gravity and presence of PMN leukocytes in the pleural fluid indicate an exudative effusion, which can be further classified as transudative or exudative based on its characteristics.

**Why the Correct Answer is Right**
The presence of a high specific gravity and large numbers of polymorphonuclear (PMN) leukocytes in the pleural fluid suggests an exudative effusion. Among the exudative effusions, purulent effusions are characterized by the presence of a high number of PMN leukocytes, indicating a bacterial infection. In this case, the patient has pneumococcal pneumonia, which is a bacterial infection, making a purulent exudate the most likely diagnosis. The high specific gravity of the pleural fluid is consistent with a high protein content, often seen in purulent effusions.

**Why Each Wrong Option is Incorrect**
**Option A:** Fibrinous exudate typically presents with a low specific gravity and a predominance of mesothelial cells, which is not consistent with the given scenario.
**Option B:** Lymphedema is not a type of pleural effusion, but rather a condition characterized by the accumulation of lymph fluid in the interstitial tissue, often due to lymphatic obstruction.
**Option D:** Serosanguineous exudate typically presents with a mixture of blood and fluid, which is not consistent with the high specific gravity and PMN leukocyte predominance in this case.

**Clinical Pearl / High-Yield Fact**
Purulent pleural effusions are often associated with bacterial infections, such as pneumococcal pneumonia, and require prompt antibiotic treatment to prevent complications.

**✓ Correct Answer: C. Purulent exudate.**