**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 clinically and laboratory-normal but have two affected children, the most plausible explanation is **germline mosaicism**. This occurs when a parent has a mixture of normal and mutated cells in their reproductive cells (sperm or eggs), but not in their somatic cells. As a result, they can pass the mutation to their offspring without expressing the disease themselves.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because if it were a simple autosomal dominant inheritance, one of the parents would likely express the disease, given its high penetrance.
**Option B:** and **Option D:** Without specific details on these options, the general principle of germline mosaicism best explains the scenario provided.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **germline mosaicism** can lead to the recurrence of autosomal dominant diseases in families where neither parent appears to be affected. This concept is crucial for genetic counseling.

**Correct Answer:** D. Germline mosaicism.