**Core Concept**
Psoriasis, lichen planus, and fungal infections are all skin conditions that can affect the nails. These conditions can lead to changes in nail appearance, including thickening, discoloration, and deformation. Understanding the relationship between these conditions and nail involvement is crucial for accurate diagnosis and treatment.

**Why the Correct Answer is Right**
All three options are correct because they can cause nail involvement. Psoriasis can lead to pitting, thickening, and discoloration of the nails, while lichen planus can cause nail thinning, brittleness, and separation from the nail bed. Fungal infections can cause nail thickening, discoloration, and crumbling. These changes occur due to the direct effect of the underlying condition on the nail matrix, nail plate, or surrounding tissues.

**Why Each Wrong Option is Incorrect**
* **Option A:** While psoriasis is a correct answer, the question is asking about nail involvement in all three options, so stating only psoriasis would be incorrect.
* **Option B:** Lichen planus is a correct answer, but stating only lichen planus would be incorrect because the question includes other options that are also correct.
* **Option C:** Fungal infection is a correct answer, but stating only fungal infection would be incorrect because the question includes other options that are also correct.

**Clinical Pearl / High-Yield Fact**
When evaluating nail changes, consider the underlying cause, as nail involvement can be a key diagnostic clue for various skin conditions. Remember the "PsOriasis, Lichen PlaNus, and Fungal Infections" (POLFI) acronym to help you remember the common causes of nail changes.

**✓ Correct Answer: D. All**