This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory, Subject name: Medicine
A single retrocardiac air-fluid level on a chest radiograph typically implies the presence of: 
Select and explain. 
A. Sliding hiatal hernia, B. Gastric volvulus, C. Pericardial effusion, D. Lung abscess
A: Sliding hiatal hernia
Answer: A: Sliding hiatal hernia 
Explanation: 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