**Question:** All are true regarding Hodgkin's lymphoma, except -

A. Hodgkin lymphoma is a type of non-Hodgkin lymphoma.
B. The disease is caused by a single genetic mutation.
C. It primarily affects children and adolescents.
D. The prognosis is always poor.

**Correct Answer:** D. The prognosis is always poor.

**Core Concept:** Hodgkin lymphoma is a type of non-Hodgkin lymphoma that primarily affects the lymphatic system. It is characterized by the presence of distinctive Reed-Sternberg cells, which are large, bizarre-looking cells that are hallmarks of the disease.

**Why the Correct Answer is Right:**

Hodgkin lymphoma is not a type of non-Hodgkin lymphoma, as stated in option A. It is a distinct entity within the broader group of lymphomas, which are cancers affecting the immune system's lymphoid cells. Hodgkin lymphoma is a specific lymphoma that involves the presence of Reed-Sternberg cells.

**Why Each Wrong Option is Incorrect:**

Option B is incorrect because Hodgkin lymphoma is caused by a complex interplay of genetic and epigenetic changes, rather than a single genetic mutation. The disease involves multiple genetic abnormalities, such as deletions, duplications, and translocations, leading to dysregulated gene expression and immune system dysfunction.

Option C is incorrect because Hodgkin lymphoma can occur at any age, although it is more common in adults. It is not primarily a disease of children and adolescents, as stated in option C. Hodgkin lymphoma accounts for a significant proportion of lymphoma diagnoses in adults but is less common in children and young adults.

Option D is incorrect because Hodgkin lymphoma's prognosis depends on several factors, including the stage of the disease, the presence of extranodal involvement, and the type of treatment received. While some patients with Hodgkin lymphoma have a favorable prognosis, others can have an unfavorable course, making option D incorrect.

**Why Each Wrong Option is Incorrect:**

Option A discusses the genetic basis of the disease, which is incorrect. The correct answer is about the specific cell type (Reed-Sternberg cells) and the disease's clinical characteristics, not its genetic underpinnings.

**Clinical Pearl:**

Understanding the clinical features and pathophysiology of Hodgkin lymphoma is crucial for diagnosing and managing patients with this disease. Adequate knowledge of the disease's cell type, clinical presentation, and treatment response will help medical professionals provide appropriate care for patients with lymphoma.