What is the most likely cause of a pleural effusion with exudative characteristics, as indicated by elevated pleural lactate dehydrogenase (LDH) and protein levels compared to serum values?

Tuberculosis is the Most Likely Cause

This pleural effusion is an exudate by Light's criteria, and tuberculosis is the most likely diagnosis among the options provided.

Application of Light's Criteria

Let me calculate the ratios from the laboratory values:

• Pleural/Serum Protein Ratio: 60/35 = 1.71 (>0.5 = exudate) 1
• Pleural/Serum LDH Ratio: 200/100 = 2.0 (>0.6 = exudate) 1
• Pleural LDH: 200 (>2/3 upper limit of normal serum = exudate) 1

All three Light's criteria are met, definitively classifying this as an exudative effusion 1, 2. The European Respiratory Society guidelines establish Light's criteria as the gold standard with 98% sensitivity for identifying exudates 1.

Eliminating Transudate Causes

The answer choices include three conditions that cause transudates, not exudates:

• Cirrhosis (Option A): Causes transudates in 3-10% of pleural effusions, characterized by low protein and LDH 1, 2
• Congestive Heart Failure (Option B): The most common cause of transudates, accounting for 29-53.5% of all pleural effusions 1, 2
• Hypoproteinemia (Option D): Causes transudates due to decreased oncotic pressure 1

These three options are immediately excluded because this patient has an exudate, not a transudate 1, 2.

Why Tuberculosis is the Answer

Tuberculosis (Option C) always causes exudative effusions 1. The European Respiratory Society explicitly states that "some pleural effusions are always exudative (e.g. tuberculosis)" 1.

Key characteristics of tuberculous pleural effusion:

• Always exudative with high protein and LDH ratios 1, 3, 4
• Accounts for 6-44% of exudative effusions depending on regional TB prevalence 1, 5, 6
• The pleural fluid/serum protein ratio typically exceeds 0.5 and LDH ratio exceeds 0.6 4, 6
• Most common in younger patients (first five decades) 5
• Typically lymphocyte-predominant, though this wasn't provided in the question 3, 6

Clinical Caveat

While the 25-30% misclassification rate of Light's criteria means some cardiac or hepatic transudates can be mislabeled as exudates 1, 7, this scenario is unlikely here given:

• All three Light's criteria are dramatically elevated (not borderline values) 1
• The pleural/serum protein ratio of 1.71 is far above the 0.5 threshold 1
• The pleural/serum LDH ratio of 2.0 is more than triple the 0.6 threshold 1

Among the four options provided, tuberculosis is the only condition that causes exudative effusions 1, 3, 6.