What is the base form of a local anesthetic that is lipid-soluble and penetrates nerve membranes?

The correct choice is the unionized form of local anesthetics. Local anesthetics exist in two forms: ionized and unionized. The unionized form, which is lipid-soluble, is capable of penetrating nerve membranes effectively. This lipid solubility allows the anesthetic to cross the phospholipid bilayer of nerve cells, where it can then exert its effects by blocking sodium channels and preventing action potentials.

In contrast, the ionized (or cation) form of local anesthetics is water-soluble, which limits its ability to diffuse through lipid membranes. While the ionized form is important for the anesthetic's action at the receptor site after it has entered the nerve, it is the unionized form that is able to penetrate the neural membrane initially.

Understanding this distinction is key in the pharmacodynamics of local anesthetics, as the ratio of unionized to ionized forms can be influenced by factors such as pH and the specific pK of the anesthetic agent used.