## **Core Concept**
Single gene disorders, also known as monogenic disorders, are conditions caused by variations or mutations in the DNA sequence of a single gene. These disorders can be inherited in an autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, or mitochondrial pattern. Understanding the genetic basis of diseases is crucial for diagnosis, management, and genetic counseling.

## **Why the Correct Answer is Right**
Retinoblastoma is a classic example of a single gene disorder. It is caused by mutations in the **RB1 gene**, which is a tumor suppressor gene. This condition leads to the development of tumors in the retina, typically in children. The inheritance pattern of retinoblastoma is autosomal dominant, meaning a mutation in one of the two copies of the gene is sufficient to cause the condition.

## **Why Each Wrong Option is Incorrect**
- **Option A: Glycogen storage disease** - While glycogen storage diseases are genetic disorders, they are typically caused by mutations in multiple different genes, each leading to a specific type of glycogen storage disease. They are not usually classified as single gene disorders in the traditional sense of being caused by a mutation in one specific gene leading to a straightforward autosomal dominant or recessive pattern.
- **Option C: DM (Diabetes Mellitus)** - Diabetes Mellitus is a complex disorder influenced by multiple genetic and environmental factors. It does not follow a simple single gene inheritance pattern but is rather considered a polygenic disorder.
- **Option D: HT (Hypertension)** - Like diabetes mellitus, hypertension is a multifactorial disorder influenced by numerous genetic and environmental factors. It does not follow a single gene inheritance pattern.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that single gene disorders often exhibit a clear family history and may follow specific inheritance patterns (e.g., autosomal dominant, autosomal recessive). Recognizing these patterns can aid in diagnosis and genetic counseling. For example, in the case of **Retinoblastoma**, if one parent has the condition, there's a 50% chance each child will inherit the mutated gene.

## **Correct Answer: B. Retinoblastoma**