## **Core Concept**
The question pertains to the field of psychiatry and genetics, specifically focusing on the heritability of major depression. Major depression, also known as major depressive disorder (MDD), is a complex condition influenced by both genetic and environmental factors. The heritability of MDD is estimated to be around 40-50%, indicating a significant genetic component.

## **Why the Correct Answer is Right**
The chance that an identical twin sister will develop major depression if the other twin has the condition is related to the concept of concordance rates in twin studies. Identical (monozygotic) twins share 100% of their genes, whereas fraternal (dizygotic) twins share on average 50% of their genes. The higher concordance rate in identical twins compared to fraternal twins for a condition indicates a stronger genetic component. For major depression, the concordance rate in monozygotic twins is approximately 40-50%. Therefore, if one identical twin has major depression, the risk that the other twin will also develop the condition is about 40-50%.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a much lower risk than what is supported by twin studies for major depression. Given the heritability and concordance rates, this option underestimates the risk.
- **Option B:** This option might seem plausible but does not accurately reflect the concordance rate for major depression in identical twins. It is closer but still not the best estimate based on current understanding.
- **Option D:** This option overestimates the risk. While genetics play a significant role, the condition is also influenced by environmental factors, making a 100% risk unlikely even in identical twins.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while there is a significant genetic component to major depression, with a heritability of about 40-50%, environmental factors also play a crucial role. This is why interventions often include both pharmacological and psychotherapeutic approaches.

## **Correct Answer:** . **40-50%**