Which disorder is primarily characterized by a strong belief of having a serious illness despite medical evidence to the contrary?

The disorder characterized by a strong belief in having a serious illness despite the lack of medical evidence is hypochondriasis, now often referred to as illness anxiety disorder. This condition involves excessive worry or preoccupation with having a serious illness, leading individuals to misinterpret normal bodily sensations as indications of disease, despite thorough medical evaluations that find no significant health issues.

In contrast, somatic symptom disorder involves real physical symptoms that are distressing and may lead to significant impairment, but the focus is on the actual symptoms rather than a belief in having a serious illness when there is no evidence. Conversion disorder involves neurological symptoms that manifest as physical symptoms but are not based on any underlying medical condition. Body dysmorphic disorder focuses on an obsessive concern with perceived defects or flaws in physical appearance, which is distinct from the concerns centered on physical health seen in hypochondriasis. Thus, the nature of the belief and the focus on illness without objective findings solidly define hypochondriasis as the correct answer in this context.