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It's a fairly typical situation. The reason many people go for an MBA is to switch industries or to jump tears, basically doing something they would not be able to achieve without the help of the MBA degree and an internship to ease into a field. For some areas, MBA is really the only or primary wait to get in. I believe about half the people goes into the bschool for the reason of switching their industry (fewer end up doing that but more on that later)
However as you correctly sensing, lacking experience is a negative when you're competing with your classmates for certain positions. For example if you want to shift from marketing into finance, it will be tough to compete for best internships and jobs with all the guys who had five or six years of experience working through the finance roles. They will trump your experience unless you have something else to offer.
Speaking of something else, Your primary tools to help jump industries will be networking (biggest asset), accomplishments in your current roles (working extra hard, learning, and initiative is what employers look for), internships and clubs (investment club or consulting club), certifications, classes, competitions (e.g. Business plan competition), and positions that's me leverage or combine your previous work experience such as Tech in finance or marketing and tech, etc.
Keep in mind that while you will not scare the admissions committee if you tell them you want to move from marketing to finance, they will be evaluating how serious and committed you are to your goals. If the sense that this is just a crazy idea or that you haven't thought through it or that it's not feasible, they don't want to admit a guy like that.
However many NBA students change their plans during the two years. For one, they realize that their initial goals are no longer relevant or the internship they took in the summer turned out to be quite a bit different than what they expected or job search proved to be too challenging and breaking into management consulting or investment banking is too hard so they fo back to a familiar industry where they have more comfort and experience.
However, as they say, nothing is impossible. It's all about your commitment and how much you're willing to sacrifice to achieve your goal.
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So say Im 30 by the time I finish B school....I want to go into McKinsey consulting, I have 0 consulting background. How likely is that to happen? How about tech as a product manager or any sort of management/product? If i go to a top 5 B school, I'm not understanding the hurdles here.