Not a routine pleural effusion
This is an image obtained from the perisplenic window in a patient with gun shot wound. At first glance, it looks like a regular pleural effusion with atelectatic lung above the spleen. If you take a closer look, a huge mass of jelly-like clotted blood can be seen in between the spleen and collapsed lung surrounded by anechoic fluid, which is most likely unclotted blood (i.e., hemothorax) in this clinical context. Sonographic image courtesy: Dr. Robert Jones.
More relevant scenario for internal medicine folks would be a patient with hypercoagulable state undergoes thoracentesis and follow up chest ultrasound demonstrates this finding. Though this degree of bleeding is uncommon, fibrin stranding is often seen in such cases, which can be confused with pleural effusion of infectious origin. Interpretation of POCUS findings in the right clinical context is the key.
Similar to the above case but a different scenario, here is an interesting image shared by Dr. Hailey Hobbs. What looks like liver initially is actually a well-circumscribed empyema!! Note the heterogeneous echotexture and moving particles in that structure. Take home message: Always slide through the area of interest instead of a ‘single point’ examination.