A 70-year-old man presents with progressive shortness of breath and intermittent dull left-sided chest discomfort for several months.

 He denies fever or acute chest pain but reports a 20-year history of rheumatoid arthritis with longstanding pleural thickening seen on previous imaging.

On examination, he is comfortable at rest, afebrile, and saturating 97 % on air.

 There is stony dullness and reduced breath sounds over the left hemithorax.

 Thoracic ultrasound confirms a large chronic pleural effusion with thickened pleural rind.

 Diagnostic aspiration yields opaque, milky fluid.

Pleural fluid analysis:

Triglyceride: 0.8 mmol/L

Cholesterol: 6.2 mmol/L

Protein: 50 g/L

Cytology: negative for malignancy

Cholesterol crystals present

According to the BTS Pleural Disease Guideline (2023), what is the most likely diagnosis