When seeing is not believing: a scope mistaken for a foreign body.

OBJECTIVES: A convincing endoscopic image can prove false when the observer's expectation supplies a conclusion before evidence is tested. We describe how a flexible ureteroscope can mimic a retained intraluminal foreign body, and how anchoring and diagnostic momentum can drive an operator to an unnecessary and hazardous retrieval. CASE PRESENTATION: During an elective flexible ureteroscopy as a planned second-look procedure, a linear, smooth, metallic-appearing structure within a dilated calyx was taken for a retained intraluminal foreign body, consistent with the residual hardware anticipated in the referring operative note. It was followed for approximately 3 cm before clinical doubt prompted reassessment. Two equipment-free manoeuvres established the truth: releasing tip deflection made the apparent foreign body move with the scope and disappear, and a guidewire passed retrogradely through a ureteric catheter emerged from the supposed forward opening, showing that the line of sight ran back along the instrument's own axis. The "foreign body" was the distal shaft of the ureteroscope, brought into view by passive buckling within the dilated collecting system. Had it been grasped, the deflected shaft would have been seized against the urothelium, exposing the patient to complications for no benefit. CONCLUSIONS: The near miss arose not from faulty optics but from anchoring and diagnostic momentum: a handover narrative primed the operator to expect retained hardware, and a high-fidelity image confirmed the expectation before it was verified. Before acting on a striking image that confirms what one was told to expect, ask what simple independent test would distinguish it from its most likely imitator.