A couple has two children affected with tuberous selerosis. On detailed clinical and laboratory evaluation (including molecular studies) both parents are normal. Which one of the following explains the two affected children in this family?
**Core Concept**
The underlying principle being tested is the genetic basis of **tuberous sclerosis**, specifically its inheritance pattern and the possibility of **germline mosaicism**. Tuberous sclerosis is an **autosomal dominant** disorder characterized by the development of benign tumors in multiple organs.
**Why the Correct Answer is Right**
Given that both parents are normal on clinical and laboratory evaluation, the most plausible explanation for having two affected children is **germline mosaicism** in one of the parents. This occurs when a mutation is present in the **germ cells** (sperm or eggs) of a parent but not in their somatic cells, leading to a risk of passing the mutation to offspring without the parent showing signs of the disease.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not account for the parents being unaffected.
**Option B:** Without the specific text of Option B, we cannot directly address why it is incorrect, but typically, such options might not correctly explain the genetic mechanism behind the scenario.
**Option C:** Similar to Option B, without the text, we can't directly refute it, but it likely fails to address the genetic principle of germline mosaicism or another relevant genetic concept.
**Option D:** This option is also incorrect as it doesn't align with the provided correct answer, which is based on the principle of germline mosaicism.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that **germline mosaicism** can lead to multiple affected offspring from apparently unaffected parents, which is a critical consideration in genetic counseling for families with a history of autosomal dominant disorders.
**Correct Answer:** D. Germline mosaicism