Final answer:
The described scenario is an example of Berksonian bias in a case-control study, where the cases and controls come from different referral patterns, leading to selection bias.
Step-by-step explanation:
The scenario described is an example of Berksonian bias, which occurs in case-control studies. In this situation, the cases (patients with disease X) are more likely to be referred to the hospital for reasons not related to the catchment area of the hospital. In contrast, the controls (patients without disease X) are primarily from the local catchment area. This difference in referral patterns can result in a selection bias because the comparison is between two groups that do not represent the same base population.