Heavy metals such as [tex]\( \text{Pb}^{2+} \)[/tex] (lead) or [tex]\( \text{Hg}^{2+} \)[/tex] (mercury) are known to disrupt various biological processes in cells. They do this by binding to enzymes or other macromolecules, and the nature of their inhibition is such that they cause permanent or long-lasting deactivation of these enzymes. This implies that the enzyme's function is permanently lost or significantly hampered until the inhibitor is removed, possibly through special treatments or not at all.
The mechanism wherein the inhibitor binds to the enzyme and causes permanent inactivation aligns with the characteristics of an irreversible inhibitor. Irreversible inhibitors form strong covalent bonds with the enzyme or change the enzyme's structure in a way that normal catalytic activity cannot be restored even after the inhibitor is removed.
In contrast, reversible inhibitors bind to enzymes in a way that can easily be undone, allowing the enzyme to quickly return to its normal function once the inhibitor is no longer present. Competitive and noncompetitive inhibitors fall into this category but do not cause permanent deactivation.
Given these details, heavy metals such as [tex]\( \text{Pb}^{2+} \)[/tex] and [tex]\( \text{Hg}^{2+} \)[/tex] act as irreversible inhibitors because they cause permanent changes to the enzyme structure or permanently block the active sites of the enzymes.
Thus, the correct answer is:
B) irreversible
