[rasheed]

I totally agree, but is it commonly asked in the exam?

The idea here is to know that all other options are incorrect and therefore you should choose DNA footprinting even if you don't know anything about it. Yes, such "exclusion" questions can be seen in the exam.

 

[rasheed]

Please explain why this is not Southwestern blotting.

In southwestern blot you spread proteins on filter paper then you add labeled DNA probe to see which protein bind to it.
This is not what's described in this question. What's described is a typical DNA footprinting technique where two samples of DNA are analyzed by adding DNA binding proteins then subjected to DNAase then you analyze the fragmentation pattern (DNA binding proteins protect DNA from lysis by DNAases). If the fragmentation pattern is identical then the two samples must be coming from the same or closely related species or individuals.

Also look at this thread for further details
http://www.usmle-forums.com/usmle-step-1-forum/972-northern-vs-southern-vs-western-blot-help.html