Final answer:
Eczema is the medical condition that best illustrates an external effect on the skin, as it can be triggered by external substances, unlike acne, psoriasis, and melanoma which have internal causes like hormonal fluctuations, immune responses, and genetic factors.
Step-by-step explanation:
The medical condition that demonstrates an external, rather than an internal effect on the skin is eczema. Eczema is an inflammatory condition and occurs in individuals of all ages, often resulting from external triggers such as allergens, irritants, and environmental factors. Unlike the other conditions listed that are more internally driven (such as acne due to clogged pores, psoriasis from immune responses, and melanoma typically caused by DNA damage due to UV exposure), eczema is often a reaction to external substances.
Skin disorders like acne and melanoma also affect the skin, but acne is commonly associated with hormonal changes and the resulting overproduction of sebum clogging pores, while melanoma is a type of skin cancer that begins in the melanocytes. Acne is especially prevalent among adolescents and can continue into adulthood, but it involves internal bodily processes rather than purely external effects. Melanoma, on the other hand, is often linked to genetic factors and exposure to ultraviolet radiation rather than a direct external irritation.