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is it necessary that ones work experience has to be relevant to the subject that they intend to pursue? or is any field of work acceptable....

If your work ex is in a relevant area, then that will have an added advantage when you negotiate with your prospective employer after an MBA.

The B Schools per se should not have any problems with your work ex and specialization as long as you have a clearly articulated career goal that provides the link for the reason for shifting industries

One of the reasons to doing an MBA would be to have a career shift and broadbase your options for the future.

Today many jobs have cross functional relevance and may be a good idea to articulate future goals and perspectives putting as much use any residual experiences. I know I am sounding confusing...but let me explain with a example. A person with experience in IT Services can choose to have finance concentration and have his new post MBA role as a IB focussed in the IT services companies. Or a person in manufacturing can choose to be a business analyst for a IT services company post MBA. So any relevance or value that the previous experience can apply to and the vlaue add the MBA can provide - can be a very valid combo.