A 30-year-old male patient came to emergency depament with chief complaint of painful skin rash. Antivirals and analgesics were prescribed but there was no improvement in the symptoms of the patient. Viral strains, therefore isolated and were found to lack viral phosphorylating enzymes. Which of the following drug is most likely to be effective in treatment of this condition?

The image shows dermatomal distribution of vesicular lesions which are suggestive of shingles/ herpes zoster. Acyclovir, Famiciclovir, valaciclovir and ganciclovir are nucleoside analogues which must be phosphorylated to nucleotide analogues in order to function. However, if viral strains are lacking in viral phosphorylating enzymes, this step is not possible. This renders these viral strains resistant to these drugs. However, Foscarnet is a pyrophosphate analogue which directly inhibits viral DNA polymerase and therefore, does not require viral phosphorylating enzymes. Another impoant drug is cidofovir, which is a nucleotide analogue and therefore, it also does not require viral phosphorylating enzymes and hence, useful in such cases.