## **Core Concept**
The question tests knowledge of common allergens that cause skin hypersensitivity reactions. Skin hypersensitivity, also known as contact dermatitis, can be caused by various substances, including metals. The immune system mistakenly identifies these substances as harmful, leading to an allergic reaction.

## **Why the Correct Answer is Right**
Nickel is recognized as the most common metal causing skin hypersensitivity. This is due to its widespread use in everyday objects such as jewelry, coins, and certain types of stainless steel. When nickel comes into contact with the skin, it can cause an immune response leading to allergic contact dermatitis, characterized by redness, itching, and blistering. The body's immune system reacts to nickel ions that have bound to skin proteins, triggering an allergic reaction.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While chromium is indeed a metal that can cause skin hypersensitivity, particularly in the context of cement and certain types of leather, it is not as commonly encountered or as frequently implicated as nickel.
- **Option B:** Cobalt can also cause allergic contact dermatitis, often in association with nickel or chromium in metal alloys. However, it is less commonly identified as the primary cause compared to nickel.
- **Option C:** This option is not provided, but based on the correct answer being , we can infer that is the correct answer and other options are incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the "Nickel Directive" implemented in the European Union, which limits the use of nickel in products that come into direct and prolonged contact with the skin, reflecting the high prevalence of nickel allergy. Clinically, patients with nickel allergy often present with dermatitis in areas where nickel-containing objects are in close and prolonged contact with the skin, such as earlobes, necklines, or around the waist.

## **Correct Answer:** . Nickel.