A cloudy pleural effusion

A 55-year-old previously fit and well woman was admitted with a 4-week history of dry cough and 1-week history of progressive breathlessness. She had never smoked, was on no regular medications and there was no history of heart disease, malignancy, rheumatological disease or tuberculosis. Physical examination revealed signs of a moderate right-sided pleural effusion but was otherwise unremarkable. Full blood count, clotting, and renal and liver function tests were normal. C reactive protein was