Which route of administration involves injecting into the top layer of the skin?

The route of administration that involves injecting into the top layer of the skin is intradermal. This method is typically used for diagnostic purposes, such as allergy testing or tuberculosis screening, where a small amount of substance is placed between the epidermis and dermis layers of the skin. The intradermal route allows for a slower absorption and is effective for substances that need to be tested for local reactions or responses.

In contrast, intramuscular administration involves injecting deeper into muscle tissue, which allows for faster absorption but is not limited to the skin's surface. Subcutaneous injection occurs in the fatty layer just beneath the skin, providing a different absorption profile and commonly used for medications that require slower, prolonged absorption. Intravenous administration is a method where drugs are injected directly into the bloodstream, leading to very rapid effects and is not related to skin layers at all. Thus, intradermal is the specific route that directly involves the skin's surface.