What action would best minimize the risk of a burn when using iontophoresis?

To minimize the risk of a burn when using iontophoresis, increasing the size of the cathode relative to the anode is a sound choice. This is because a larger cathode disperses the current over a greater surface area, which reduces the current density at any given point on the skin. A lower current density helps to minimize thermal effects, such as burns, as less electrical energy is concentrated in any one area of the skin.

Using a larger cathode can help ensure that the skin does not become overly heated during treatment because it reduces the likelihood of generating excessive heat at the site of application. Additionally, the cathode's configuration can influence the distribution of ions and the overall effectiveness of the treatment. Thus, selecting a cathode with a larger surface area compared to the anode not only enhances safety but can also improve therapeutic outcomes in iontophoresis.

The other actions listed, such as reducing the distance between electrodes or increasing current density, would likely increase the risk of burns instead of minimizing it. Additionally, decreasing the moisture of the electrodes could reduce conductivity, potentially leading to inefficient treatment rather than improved safety.