**Core Concept**
The patient's presentation is suggestive of a chronic nail condition characterized by discoloration, thickening, and tunneling of the nails. This condition is often associated with systemic diseases, particularly those affecting the immune system, such as psoriasis, or can be idiopathic.

**Why the Correct Answer is Right**
The examination of the nail bed for pitting is a hallmark feature of psoriatic nail disease. Pitting occurs due to the accumulation of keratin debris in the nail plate, which is a result of the abnormal keratinization process seen in psoriasis. The presence of pitting, particularly in the absence of other nail changes, can be a diagnostic clue to psoriasis. In this case, the examination of the nail bed for pitting can help clinch the diagnosis at the earliest.

**Why Each Wrong Option is Incorrect**
* **Option A:** Onycholysis refers to the separation of the nail plate from the nail bed, which can be seen in various conditions, including fungal infections and psoriasis. However, it is not a specific diagnostic feature that can help clinch the diagnosis of psoriasis.
* **Option B:** Clubbing of the nails is a feature of various systemic conditions, including heart disease, lung disease, and certain infections. However, it is not a characteristic feature of psoriasis.
* **Option C:** Koilonychia refers to the thinning and spooning of the nails, which is often seen in iron deficiency anemia. However, it is not a diagnostic feature of psoriasis.

**Clinical Pearl / High-Yield Fact**
In patients presenting with nail changes, it is essential to examine the nail bed and matrix for pitting, as it can be a diagnostic clue to psoriasis or other systemic conditions.

**Correct Answer:** C.