Which of the following conditions is characterized by a thickening of the skin?

Acanthosis nigricans is characterized by a thickening and darkening of the skin, often seen in body folds such as the neck, armpits, and groin. This condition typically presents as velvety, pigmented areas that can develop due to various factors, including insulin resistance, obesity, hormonal disorders, or certain medications. The thickened skin in acanthosis nigricans is a direct result of hyperkeratosis and epidermal thickening, which differentiates it from other conditions.

In contrast, psoriasis is characterized by the presence of red, scaly patches that can also result in thickened skin, but the hallmark is the rapid turnover of skin cells leading to the plaques rather than a solely thickened appearance. Eczema can involve thickened skin, particularly when it becomes chronic, but it primarily presents with inflammation, itchiness, and redness. Fungal infections can lead to skin changes, including scaling and erythema, but do not typically cause the distinct thickening that is characteristic of acanthosis nigricans. Therefore, when considering skin thickening specifically, acanthosis nigricans is the most accurate choice.