**Core Concept**
Contact dermatitis is a type of skin inflammation caused by exposure to an irritant or allergen, leading to a delayed-type hypersensitivity reaction. This condition is characterized by a complex interplay between immune cells, cytokines, and the release of various chemical mediators.

**Why the Correct Answer is Right**
The diagnosis of contact dermatitis is primarily done by identifying the causative agent and assessing the patient's immune response. This is typically achieved through a combination of clinical evaluation, patch testing, and sometimes, biopsy. Patch testing involves applying small amounts of potential allergens to the patient's skin and observing for a reaction. If a reaction occurs, it indicates that the patient is allergic to the substance. Biopsy may be performed to rule out other conditions and to confirm the presence of a hypersensitivity reaction.

**Why Each Wrong Option is Incorrect**
**Option A:** Skin prick test is not the primary method for diagnosing contact dermatitis, as it is more commonly used for diagnosing allergic rhinitis and asthma.
**Option B:** Blood tests may be used to support the diagnosis, but they are not the primary method for diagnosing contact dermatitis.
**Option C:** Clinical evaluation alone may not be sufficient to diagnose contact dermatitis, as it can be confused with other skin conditions.

**Clinical Pearl / High-Yield Fact**
A classic example of contact dermatitis is poison ivy dermatitis, which occurs when the urushiol oil in poison ivy plants comes into contact with the skin, leading to a severe allergic reaction.

**Correct Answer: D. Patch testing and biopsy.**