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: Surgery
Solitary hypoechoic lesion of the liver without septa or debris is most likely to be - 
Select and explain. 
A. Hydatid cyst, B. Carolis disease, C. Liver abscess, D. Simple cyst
D: Simple cyst
Answer: D: Simple cyst 
Explanation: A solitary hypoechoic lesion without any septae or debris is most likely to be a simple cyst. However rarely a hydatid cyst can have a similar sonologic appearance.
Simple cysts are the most common benign lesions found in liver. On imaging studies they appear as unilocular, homogenous fluid-filled structures with a thin wall without projections, septa or debris.

Ultrasound appearances of -
Hydatid cyst -
It presents either as a simple cyst with hydatid sand or daughter cysts or floating endocyst or calcification.
Caroli's disease -
Its the type V choledochal cyst
Multiple intrahepatic biliary cysts are seen on ultrasound. Sludge & stones may also be visible within the cysts.
Liver abscess -
Sonographic appearance may include a cystic lesion with floating internal echoes and irregular margins.
Septations, debris and fluid-fluid interfaces may also be seen.