**Core Concept**
Hypochondriasis, also known as illness anxiety disorder, is a psychiatric condition characterized by excessive worry about having a serious illness despite medical reassurance and a lack of evidence to support these fears. This condition is often associated with somatic symptoms, such as headaches, and can significantly impair daily functioning.

**Why the Correct Answer is Right**
The patient's persistent fear of having a serious illness despite normal investigations and medical reassurance is indicative of hypochondriasis. This condition is often driven by an excessive preoccupation with bodily sensations and a fear of catastrophic outcomes. The patient's insistence that there is something wrong in her head, despite multiple consultations and normal investigations, further supports this diagnosis. Hypochondriasis is a type of somatoform disorder, which involves the presence of physical symptoms that cannot be explained by a medical condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Phobia involves an irrational fear of a specific object or situation, which does not align with the patient's concerns about having a serious illness.

**Option B:** Psychogenic headache refers to headaches that are caused by psychological factors, such as stress or anxiety, but this term does not capture the patient's underlying fear of having a serious illness.

**Option D:** Depression involves a persistent feeling of sadness, hopelessness, and a lack of interest in activities, which may be present in hypochondriasis but is not the primary diagnosis in this case.

**Clinical Pearl / High-Yield Fact**
Hypochondriasis often co-occurs with other psychiatric conditions, such as anxiety disorders and depression, and treatment typically involves a combination of cognitive-behavioral therapy and medication to manage symptoms.

**Correct Answer:**
✓ Correct Answer: C. Hypochondriasis