During bladder ultrasound, what indicates a urolith?

In bladder ultrasound imaging, the identification of a urolith, or urinary stone, is indicated by the presence of a black shadow obscuring underlying structures. This phenomenon is known as acoustic shadowing. When ultrasound waves encounter a dense structure like a stone, they do not pass through it but instead reflect off its surface, creating an area devoid of echoes, which appears as a dark shadow on the ultrasound image. This distinctive shadow helps radiologists and clinicians differentiate a stone from other potential findings in the bladder.

The other answer choices do not fulfill the criteria for identifying a urolith. Free fluid may be indicative of various conditions but does not specifically point to the presence of stones. A bladder wall tumor would typically present with different ultrasound characteristics, such as irregularities or thickening of the bladder wall, rather than the acoustic shadowing seen with stones. The round ligament of the bladder is an anatomical structure and is not diagnostic of urolithiasis. Thus, the presence of a black shadow is the key diagnostic indicator for uroliths during a bladder ultrasound.