Resistance to zidovudine develops due to-

Ans. is 'a' i.e., Mutations at reverse transcriptase "Resistance to Zidovudine occurs by point mutation which alters reverse transcriptase enzyme. In the past when AZT was used alone, > 50% patients became nonresponsive to AZT within 1-2 years therapy due to growth of resistant mutants" - KDT