**Core Concept**
Light's criteria are used to differentiate between exudative and transudative pleural effusions. Exudative pleural effusions are characterized by a high protein content, high lactate dehydrogenase (LDH) levels, and a high ratio of pleural fluid to serum protein and LDH. Adenosine deaminase (ADA) is also used as a diagnostic marker, particularly in tuberculous pleural effusions.

**Why the Correct Answer is Right**
The correct answer, **D. Pleural fluid ADA to Serum ADA <16**, is based on the ADA levels in the pleural fluid and serum. ADA levels are typically elevated in tuberculous pleural effusions, and a ratio of pleural fluid ADA to serum ADA of less than 16 is considered normal. This is because ADA is an enzyme that plays a crucial role in the breakdown of adenosine, and its levels are usually low in the serum. Therefore, a low ratio indicates a low ADA activity in the pleural fluid, suggesting a transudative pleural effusion.

**Why Each Wrong Option is Incorrect**
**Option A:** A pleural fluid protein to serum protein ratio of greater than 0.5 is actually one of the criteria for exudative pleural effusions, not transudative.

**Option B:** A pleural fluid LDH to serum LDH ratio of greater than 0.6 is a criterion for exudative pleural effusions, not transudative.

**Option C:** A pleural fluid LDH level of more than 2/3 times the normal upper limit for serum is another criterion for exudative pleural effusions, not transudative.

**Clinical Pearl / High-Yield Fact**
When interpreting pleural fluid ADA levels, it's essential to remember that a ratio of pleural fluid ADA to serum ADA of less than 16 is normal, and a high ratio suggests an exudative pleural effusion, particularly in the context of tuberculosis.

**Correct Answer:**
✓ Correct Answer: D. Pleural fluid ADA to Serum ADA <16