1⟩ Have you applied to any other areas apart from marketing?
If the answer is yes (and it is quite reasonable for it to be so!) then have a reason for your interest in the other areas. Some other occupations are obviously related, such as Sales, Market Research, Advertising, Promotional work, Public Relations etc.
Show that there are common skills between the other occupations and Marketing. Alternatively demonstrate that the attraction is different and perhaps of a lesser degree than Marketing. Show that the job in question attracts you the most and that there is no sense of you being inconsistent in your applications. The other companies you have applied to will interest your interviewer. Let them see that you have researched the field well. But make sure that they know they are the first choice. You do not want to put them off!