Which one of the following options is an example of a direct cross-infection incident?

A dirty inoculation injury is a clear example of a direct cross-infection incident because it involves the transfer of pathogens directly into the body through a breach in the skin. This type of incident poses a significant risk in a dental setting, especially when needlestick injuries occur or when tools used in procedures come into contact with infected bodily fluids. The injury creates a direct pathway for infectious agents to enter the bloodstream, leading to potential transmission of diseases.

In contrast, the other options involve indirect or less immediate forms of potential pathogen transmission. For instance, contact with a dirty sink tap does not necessarily involve direct entry of pathogens into the body. Inhalation of aerosol in a clinical area refers to airborne pathogens, which is also an indirect form of exposure, while reusing a decontaminated paste filler relates to cross-contamination risks rather than a direct incident of infection. Thus, the dirty inoculation injury stands out as the most direct example of cross-infection.