A 35-year-old man complains of a cough and is sent for a CXR by his GP. This demonstrates a solitary cystic structure within the left lower lobe, measuring approximately 6 cm in diameter. The peripheral aspect of the cystic structure lies in contact with the chest wall and appears slightly flattened. Within this structure there appears to be a floating membrane. What is the most likely diagnosis?
Hydatid cyst rupture can lead to a variety of appearances: an air-fluid level, a floating membrane (the water lily sign), crumpled membranes in the dependent part of the cyst and a cyst with all contents expectorated (empty cyst sign).