The main reason why antibiotics could not solve all the problems of bacteria mediated diseases is-
A. Insensitivity of the individual following prolonged exposure to antibiotics
B. Inactivation if antibiotics by bacterial enzymes
C. Decreased efficiency of the immune system
D. The development of mutant strains resistant to antibiotics.

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Hint:Antibiotics are the medicines that inhibit the growth of bacteria and thus the infections caused by them. The term antibiotic itself means “against life “They are able to do this either by killing them or by inhibiting them from copying themselves. Any drug that we take in order to kill the germs is termed as an antibiotic.

Complete answer: In order to answer this question, we need to understand what an antibiotic does when it enters our body, i.e., its mechanism of action.
The dose or the type of antibiotic that we use is based on the type of bacteria that has infected our body. Our body is not insensitive to the doses of antibiotics used. A major problem that we face while treating bacterial mediated diseases is that many bacteria have been found to show resistance to antibiotics. The development of antibiotic-resistant bacteria is closely related to the extent that antibiotics have been used in the human body. Due to the prolonged use of antibiotics, the bacteria become resistant to it in due course of time. These resistant strains may grow rapidly or slowly. It depends upon the amount or type of antibiotic used. Bacteria occur in a large amount and thus there is a high probability of development of a mutant individual in the population. As soon as the mutant individuals develop, it starts exhibiting the resistant properties, and eventually multiplies its number and becomes the dominant type.

So, the correct option is D.

Note: Antibiotics are not effective for viral diseases. These powerful medicines have the ability to fight bacterial infections. Viruses grow and multiply in the host body, by using the organism's machinery organs. So, it becomes difficult for the antibodies to aim the viruses without affecting the host. They contain genetic material, either RNA or DNA.