**Core Concept**
Koenen's periungual fibroma is a type of benign growth that occurs around the nails, specifically in the periungual region. It is a characteristic feature of a genetic disorder.

**Why the Correct Answer is Right**
Koenen's periungual fibroma is a hallmark of tuberous sclerosis complex (TSC), a rare genetic disorder that affects multiple organ systems. The growths are caused by the abnormal growth of connective tissue, leading to the formation of fibromas around the nails. The pathogenesis of Koenen's periungual fibroma involves the activation of the mTOR pathway, leading to the proliferation of fibroblasts and the formation of fibromas.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Koenen's periungual fibroma is not typically seen in psoriasis, although psoriasis can cause nail changes.
**Option B:** This option is incorrect because Koenen's periungual fibroma is not a feature of alopecia areata, which is an autoimmune disease that affects the hair follicles.
**Option C:** This option is incorrect because Koenen's periungual fibroma is not a feature of eczema, although eczema can cause changes to the skin and nails.

**Clinical Pearl / High-Yield Fact**
Koenen's periungual fibroma is a key diagnostic feature of tuberous sclerosis complex, and its presence should prompt a thorough evaluation for other manifestations of the disease.

**Correct Answer: D. Tuberous sclerosis complex.**