On a chest X-ray, which of the following signs indicates that a pleural effusion has a loculated (encapsulated) component rather than a freely flowing one?

• A Meniscus sign with blunting of the costophrenic angle
• B Fluid tracking into the minor fissure
• C Subpulmonary effusion with an apparent elevated hemidiaphragm
• D D-shaped opacity on the PA view that does not shift on lateral decubitus ✓

Explanation

A D-shaped (lenticular) pleural opacity that does not change position on decubitus views indicates a loculated (encapsulated) effusion, as free fluid redistributes with gravity. The meniscus sign and fissure tracking are features of free fluid. A subpulmonary effusion simulates an elevated hemidiaphragm but shifts on decubitus positioning, confirming free fluid.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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