Final answer:
An 'apple core' sign on an x-ray is a classic indication of colorectal cancer, observed during a barium enema study and usually warrants further investigation like a colonoscopy.
Step-by-step explanation:
The term "apple core" on an x-ray refers to a classic radiographic sign of colorectal cancer, specifically describing the appearance of a stenotic lesion in the colon that looks similar to an apple core. This sign is typically seen on a barium enema study. Patients with this finding usually require further investigation, like a colonoscopy, to confirm the diagnosis and to allow for biopsy of the lesion. Although colon cancer can manifest in various forms, the "apple core" lesion is one of the more distinctive radiographic signs prompting urgent medical evaluation.