## Core Concept
Nail psoriasis is a common manifestation of psoriasis, a chronic autoimmune skin condition. The condition affects the nails in a significant proportion of patients, leading to various nail changes. Understanding the prevalence of nail involvement is crucial for diagnosing and managing psoriasis.

## Why the Correct Answer is Right
The correct answer indicates that nail changes are found in about 50% of cases of psoriasis. This high prevalence is due to the fact that psoriasis can affect any part of the nail unit, including the nail plate, nail bed, and nail matrix. The nail changes can be due to the direct effect of psoriatic inflammation on the nail unit or due to the systemic inflammation associated with psoriasis.

## Why Each Wrong Option is Incorrect
* **Option A:** 10% is significantly lower than the actual prevalence of nail changes in psoriasis. While some cases may be mild and not immediately noticeable, 10% underestimates the true frequency.
* **Option B:** 20% is still lower than the commonly reported prevalence. Although it might reflect milder cases or those not seeking medical attention, it does not accurately represent the overall frequency of nail involvement.
* **Option D:** 90% overestimates the prevalence of nail changes in psoriasis. While nail involvement is common, not all patients with psoriasis develop nail changes.

## Clinical Pearl / High-Yield Fact
A key point to remember is that nail psoriasis can precede skin symptoms in some cases, and the presence of nail changes can significantly impact a patient's quality of life. Nail involvement is also associated with more severe disease and a higher likelihood of psoriatic arthritis.

## Correct Answer: C. 50%