**Core Concept**
Air-borne contact dermatitis (ACD) is a type of allergic contact dermatitis caused by an immune response to airborne allergens, such as dust mites, pollen, or pet dander. This condition results in skin inflammation and lesions, often accompanied by pruritus (itching).

**Why the Correct Answer is Right**
The patch test is a diagnostic tool used to identify allergens responsible for contact dermatitis, including ACD. A patch test involves applying small amounts of potential allergens to the skin and observing for a reaction after 48-72 hours. In the case of ACD, the patch test can help identify airborne allergens that trigger the immune response, leading to skin symptoms. This is because the allergen must come into direct contact with the skin to induce a reaction, making the patch test an effective method for diagnosing ACD.

**Why Each Wrong Option is Incorrect**
**Option A:** Skin biopsy is a diagnostic procedure that involves removing a small sample of skin tissue for histopathological examination. While a skin biopsy can help diagnose various skin conditions, it is not the primary method for diagnosing ACD, which is typically diagnosed through clinical history and patch testing.

**Option C:** Prick test, also known as a skin prick test, is used to diagnose allergies, particularly respiratory allergies such as hay fever or asthma. This test involves pricking the skin with a small amount of an allergen and observing for a localized reaction. However, prick tests are not typically used to diagnose ACD, as they are not as effective in identifying contact allergens.

**Option D:** Estimation of serum IgE levels can help diagnose allergies by measuring the levels of IgE antibodies in the blood. Elevated IgE levels can indicate an allergic response, but this test is not specific for ACD and cannot identify the allergen responsible for the condition.

**Clinical Pearl / High-Yield Fact**
When considering a diagnosis of ACD, remember that the allergen must come into direct contact with the skin to induce a reaction. This is why patch testing is a crucial diagnostic tool for identifying airborne allergens that trigger ACD.

**✓ Correct Answer: B. Patch test**