What term describes discoloration in the shape of a horseshoe on the neck?

The term that describes discoloration in the shape of a horseshoe on the neck is poikiloderma. This condition typically manifests as a combination of symptoms including skin thinning, atrophy, and distinct color changes that can present in a horseshoe pattern, especially on sun-exposed areas such as the neck. Poikiloderma is often associated with conditions like photoaging or certain genetic disorders and reflects changes in skin pigmentation and texture.

In contrast, dermatitis refers to inflammation of the skin, which is characterized by redness, swelling, and irritation but does not specifically indicate a horseshoe shape or discoloration. Chelitis refers to inflammation of the lips, typically presenting as cracking or dryness, while neurodermatitis is a condition resulting from scratching due to itchiness, leading to thickened patches of skin but it does not correspond with the specific discoloration described in the question.