The 17-letter answer is:
How to get this? Well, notice that:
1. There are 17 purple lines in the fingerprint image - the same as the number of letters in the answer to be found.
2. Regardless of where each of these purple lines finishes, their left-hand ends each begin on the left-hand margin of the fingerprint.
3. The purple lines all occur within a range of 25 'rows' in the diagram.
Altogether this suggests that:
Each purple line equates to a particular letter of the alphabet, and moreover that if we label the first purple line from the top as 'A', then (proceeding down the left-hand edge) label the one below it as 'B', and so on down to the 25th in sequence (the lowest purple line) being 'Y' we will find that the 17 letters indicated by the purple lines are: ABCDExGHIxxLMxOPxRSTUxxxY.
Now, to get the final answer:
Apply the anagram tag, and produce the 17-letter word SUBDERMATOGLYPHIC. What does it mean? Well, according to encyclo.co.uk:
"Subdermatoglyphic: The set of patterns that are the fine whorls, arches, and other finger-ridges that have an underlying dermal matrix which determines their distribution."
Given the choice of image used to conceal this word, it seems rather appropriate, don't you think?
And the title?
'SUBDERMATOGLYPHIC' may very well be the longest word you can create in the English language where every letter is unique! (i.e. the longest 'isogram' in English...)