The process that takes place in the step A in the figure above is mediated by CYP3A4 enzyme

The metabolite which results from the step A above is usually more water soluble than the metabolite that is produced from the step B above

The letter B in the figure above represents the phase 1 reaction step in the metabolism of phenytoin

For the step A in the figure above, preparatory reaction such as oxidation, reduction or hydrolysis usually take place

The letter A in the figure above represents the phase 1 reaction step in the metabolism of phenytoin