**Core Concept**
Koenen's tumor, also known as periungual fibroma, is a rare benign tumor that affects the periungual region of the nail. It is a type of fibroma that arises from the connective tissue surrounding the nail plate.

**Why the Correct Answer is Right**
Koenen's tumor is a characteristic feature of tuberous sclerosis complex (TSC), a genetic disorder that affects multiple organ systems. The tumor is thought to arise from the abnormal proliferation of cells in the periungual region, which is a hallmark of TSC. The presence of Koenen's tumor is a key diagnostic criterion for TSC, and it is often used as a biomarker for the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Koenen's tumor is not typically associated with other genetic disorders, such as neurofibromatosis or von Hippel-Lindau disease.
**Option B:** Koenen's tumor is not a type of skin cancer, such as melanoma or basal cell carcinoma.
**Option C:** Koenen's tumor is not a type of nail plate abnormality, such as onycholysis or nail psoriasis.

**Clinical Pearl / High-Yield Fact**
Koenen's tumor is a key diagnostic feature of tuberous sclerosis complex, and its presence should prompt a thorough evaluation for other manifestations of the disease, including seizures, intellectual disability, and renal angiomyolipomas.

**Correct Answer: D. Tuberous sclerosis complex (TSC)**