This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Air crescent sign in chest X-ray is seen in: September 2010 
Select and explain. 
A. Blastomycosis, B. Pneumonia, C. Invasive aspergillosis, D. Bronchiectasis
C: Invasive aspergillosis
Answer: C: Invasive aspergillosis 
Explanation: Ans. C: Invasive Angioinvasive aspergillosis is the most severe and aggressive form of invasive aspergillosis and is seen only in profoundly immunocompromised patients and is a life threatening condition requiring prompt treatment. Typical appearances are those of solitary or multiple pulmonary nodules or masses. Wedge-like areas of ill-defined opacity may also be seen most likely representing infarcts due to invasion of proximal pulmonary vessels. An air crescent may be visible, although it is seen earlier on CT.