**Core Concept:** Contact dermatitis is a skin condition characterized by inflammation due to the direct contact with an irritant or allergen. The correct method to diagnose contact dermatitis involves patch testing.

**Why the Correct Answer is Right:**
Patch testing is the gold standard for diagnosing contact dermatitis, as it helps identify the specific allergen causing the reaction. In patch testing, a series of standardized allergen patches are applied to the patient's skin for 48-72 hours, then examined for reaction. This method allows for a more accurate diagnosis and helps guide treatment.

**Why Each Wrong Option is Incorrect:**
A. **Irritant contact dermatitis (ICD)** is a type of contact dermatitis, but applying irritants directly to the skin would not accurately identify the allergen causing the reaction.
B. **Pharmacodynamic testing** measures the effect of drugs on the body, not relevant to diagnosing contact dermatitis.
C. **Patch testing with irritants** would not accurately identify the allergen causing the reaction, as it would test for irritants rather than allergens.
D. **Excisional biopsies** involve removing a small piece of skin, which is invasive and not suitable for diagnosing contact dermatitis.

**Clinical Pearl:**
Contact dermatitis can be challenging to diagnose; patch testing helps in identifying the causative allergen, enabling targeted treatment options. Always consider patch testing in suspected cases of contact dermatitis to provide accurate diagnosis and treatment.

**Correct Answer:** **D.** Patch testing is the diagnostic method of choice for contact dermatitis, helping identify the specific allergen causing the reaction.