**Question:** A 20-year-old man suffers from schizophrenia. He has a monozygotic twin brother. Based on the findings from genetic studies, what is the risk (in %) that his brother will develop schizophrenia?

A. 20%
B. 40%
C. 50%
D. 80%

**Core Concept:** Genetic Studies in Schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with both genetic and environmental factors contributing to its development. Monozygotic twins (identical twins) share 100% of their genes, whereas dizygotic twins (fraternal twins) share around 50% of their genes. Genetic studies have helped determine the heritability of schizophrenia.

**Why the Correct Answer is Right:** Schizophrenia is primarily a genetic disorder, with heritability estimates ranging from 40-60%. The risk for schizophrenia in a monozygotic twin depends on the degree of genetic similarity. In this case, the monozygotic twin shares 100% of his genes with the affected individual, making him at high risk of developing schizophrenia.

**Why Each Wrong Option is Incorrect:**

A. 20%: This value represents the risk for schizophrenia in dizygotic twins but not monozygotic twins, who share 100% of their genes.

B. 40%: This value is too low, as monozygotic twins share 100% of their genes, increasing the risk significantly.

C. 50%: This value represents the risk for schizophrenia in dizygotic twins, not monozygotic twins who share 100% of their genes.

D. 80%: This value is too high, as monozygotic twins share 100% of their genes, increasing the risk significantly.

**Clinical Pearl:** The genetic component of schizophrenia is essential in understanding the risk for affected relatives. The presence of a first-degree relative with schizophrenia increases the risk of developing the disorder in another member of the family by about 10-20%, depending on the degree of genetic similarity. In this case, the monozygotic twin shares 100% of their genes, making them at high risk of developing schizophrenia.