**Core Concept**
Hypochondriasis is a psychiatric disorder characterized by excessive worry and fear about having a serious illness, despite medical reassurance and evidence to the contrary. This condition is also known as illness anxiety disorder and is distinct from somatic symptom disorder.

**Why the Correct Answer is Right**
Shailza's persistent belief in having a brain problem despite normal investigations and multiple consultations is indicative of hypochondriasis. Her preoccupation with her perceived illness and the lack of relief from reassurance from medical professionals are hallmarks of this disorder. Hypochondriasis is often associated with anxiety and depression, but in this case, the primary symptom is the excessive worry about having a serious illness.

**Why Each Wrong Option is Incorrect**
**Option A:** Acute mania is a state of abnormally elevated mood, energy, and decreased need for sleep, which does not align with Shailza's symptoms of headache and preoccupation with illness.

**Option B:** Acute depression is a mood disorder characterized by persistent sadness, loss of interest, and changes in appetite or sleep, which is not the primary presentation in this scenario.

**Option C:** Psychogenic headache refers to headaches with a psychological origin, but Shailza's symptoms are more related to her fear of having a serious illness rather than the headache itself.

**Clinical Pearl / High-Yield Fact**
Hypochondriasis is often associated with a need for reassurance and a preoccupation with checking one's body for signs of illness. This can lead to excessive medical consultations, tests, and treatments, which can be costly and time-consuming for the patient and the healthcare system.

**✓ Correct Answer: D. Hypochondriasis**