**Core Concept**
The patient's belief that his penis is decreasing in size and will eventually disappear, leading to death, is a classic example of a body image disturbance, specifically a type of delusional disorder known as a somatic delusion. This condition involves a persistent, unfounded belief that one's body is changing or is affected in some way, often with catastrophic consequences.

**Why the Correct Answer is Right**
The patient's delusional disorder is characterized by a fixed, false belief that his penis is shrinking and will eventually disappear, leading to death. This delusional thinking is often resistant to evidence or reasoning to the contrary and can be a source of significant distress and impairment in social and occupational functioning. The patient's symptoms are not better explained by another mental disorder, such as obsessive-compulsive disorder or body dysmorphic disorder, and are not due to the effects of a substance or another medical condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the patient's symptoms are not a manifestation of a psychotic disorder, such as schizophrenia, which would involve a broader range of symptoms, including hallucinations, disorganized thinking, and negative symptoms.

**Option B:** This option is incorrect because the patient's symptoms are not a manifestation of a mood disorder, such as major depressive disorder, which would involve a persistent depressive mood, loss of interest in activities, and changes in appetite or sleep.

**Option C:** This option is incorrect because the patient's symptoms are not a manifestation of a anxiety disorder, such as generalized anxiety disorder, which would involve excessive worry, anxiety, and avoidance behaviors.

**Clinical Pearl / High-Yield Fact**
Delusional disorders, including somatic delusions, are often associated with significant distress, impairment in social and occupational functioning, and a poor response to treatment. A thorough diagnostic evaluation, including a mental status examination and a review of the patient's medical and psychiatric history, is essential in differentiating delusional disorders from other psychiatric conditions.

**Correct Answer:** D. Delusional disorder, somatic type