**Core Concept**
Tinea unguium is a fungal infection of the nail plate, caused by dermatophytes, which invade and destroy the keratin in the nail. This leads to changes in the nail's appearance, such as thickening, discoloration, and brittleness.

**Why the Correct Answer is Right**
The correct answer is Trichophyton rubrum, a common dermatophyte that causes Tinea unguium. This fungus invades the nail plate through a crack or break in the nail, and then begins to degrade the keratin, leading to the characteristic changes in the nail's appearance. The infection can spread to other nails, and in severe cases, can lead to pain and disability.

**Why Each Wrong Option is Incorrect**
* **Option A:** Epidermophyton floccosum is another dermatophyte that causes skin infections, but it is not typically associated with Tinea unguium.
* **Option B:** Microsporum canis is a dermatophyte that primarily causes skin infections in animals, and is not typically associated with Tinea unguium in humans.
* **Option C:** Aspergillus is a type of fungus that can cause nail infections, but it is not a dermatophyte and is not typically associated with Tinea unguium.

**Clinical Pearl / High-Yield Fact**
Tinea unguium is more common in older adults, and can be difficult to treat due to the slow growth rate of the nail. Early diagnosis and treatment are essential to prevent the spread of the infection to other nails.

**Correct Answer:** D. Trichophyton rubrum.