## Core Concept
The question revolves around the diagnosis of a patient presenting with persistent complaints of headache despite normal investigations. This scenario suggests a condition where the patient experiences symptoms that are not fully explained by a general medical condition. The key concept here is to identify a condition characterized by excessive worry about having a serious illness despite medical reassurance and normal diagnostic tests.

## Why the Correct Answer is Right
The correct answer, **D.**, points towards a diagnosis of "Body Dysmorphic Disorder" or more accurately in this context, "Health Anxiety" or "Hypochondriasis." This condition is characterized by excessive worry about having a serious illness despite medical reassurance and normal diagnostic tests. Patients often believe they have a specific disease or condition and are preoccupied with the fear of having or the idea that they have a serious, undiagnosed medical condition. This worry significantly interferes with their daily life.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, conditions like tension headaches or migraines would be considered if there were clear indications of such (e.g., specific headache characteristics, triggers).
- **Option B:** Similarly, not provided, but one might expect a condition like depression or anxiety disorder to be considered. While these can present with physical complaints, the specific focus on fear of illness and request for repeated medical evaluations despite normal findings steers away from a primary diagnosis of depression or a generalized anxiety disorder.
- **Option C:** Without specifics, one might guess this could represent another psychiatric or medical condition. However, given the context, conditions that do not primarily involve a preoccupation with having a serious undiagnosed illness are less likely.

## Clinical Pearl / High-Yield Fact
A key point to remember is that patients with **Health Anxiety** or **Hypochondriasis** often do not respond well to simple reassurance. Their concern about illness is not easily alleviated by medical evaluation or reassurance, and they may seek multiple medical opinions. Management involves a combination of cognitive-behavioral therapy (CBT) and sometimes selective serotonin reuptake inhibitors (SSRIs).

## Correct Answer: D. Hypochondriasis.