<prompt> A single retrocardiac air-fluid level on a chest radiograph typically implies the presence of:Sliding hiatal herniaGastric volvulusPericardial effusionLung abscess  <response>: Ref: Canadian Asso Radiology journal, 1986Explanation:Sliding hiatus hernia is the most common type of hiatus herniaThe GE junction is displaced upwardsThe gastric fundus is also displaced above the diaphragm - retrocardiac mass with air fluid levelA differential 2 airfluid levels at different interfaces retrocardiac air-fluid level - gastric volvulus.Pericardial effusion produces water bottle appearance of cardiac silhouette on CXRLung abscess produces a cavity with airfluid level 