Final answer:
The most appropriate diagnosis for the patient convinced they have cancer without evidence is Illness Anxiety Disorder, characterized by concern over having a serious undiagnosed illness causing significant distress.
Step-by-step explanation:
The most appropriate diagnosis for a patient who is convinced they have cancer despite a full workup with negative results and reassurance from doctors is Illness Anxiety Disorder. This condition is characterized by the patient's preoccupation with having a serious, undiagnosed medical illness despite lack of substantial evidence. Illness Anxiety Disorder often results in significant distress and impairment in social, occupational, or other important areas of functioning. It is distinguished from Somatic Symptom Disorder by the absence of severe physical symptoms. The patient does not manifest a neurological condition such as a stroke or epilepsy, which can be tested and demonstrated through clinical examination and testing.
Conditions like Conversion Disorder and Body Dysmorphic Disorder are not applicable in this case as there are no neurological symptoms indicative of a conversion disorder, nor is there an excessive concern with appearance that would suggest body dysmorphic disorder. Briquet's Syndrome, also known as Somatization Disorder, typically involves multiple physical complaints over many years, which is not detailed in the patient's scenario provided.