**Core Concept**
Pitting of nails is a clinical sign characterized by small depressions on the surface of the nails, often associated with nail plate thinning. It is a manifestation of nail matrix damage, which can be caused by various underlying conditions.

**Why the Correct Answer is Right**
Pitting of nails is commonly seen in psoriasis, an autoimmune disease that affects the skin and nails. The nail matrix, responsible for nail growth, is affected by the inflammatory process in psoriasis, leading to the formation of pits. The exact mechanism involves the abnormal activation of matrix keratinocytes, resulting in the disruption of normal nail plate formation.

**Why Each Wrong Option is Incorrect**
**Option A:**
This is incorrect because while nail pitting can be associated with other conditions like alopecia areata, it is not the most common or specific association.

**Option B:**
This is incorrect because nail pitting is not typically seen in conditions like onychomycosis, which affects the nail plate directly.

**Option C:**
This is incorrect because while nail abnormalities can be seen in conditions like rheumatoid arthritis, pitting of nails is not a characteristic feature.

**Clinical Pearl / High-Yield Fact**
One should remember that nail pitting is a specific sign that can help differentiate psoriasis from other skin conditions. It is essential to consider the clinical context and other associated features to arrive at a correct diagnosis.

**Correct Answer:** D.