**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