I'm still not fully convinced with the paragraph (many flaws). But, since you asked particularly about those words and the articles, I'm addressing that so that to clarify that *in general*. Maybe, you take care of this in future then.  

In this case, you are talking about the challenge of **lacking** something. Thus, it's the *challenge* and not *two or more* challenges!

In any case, *[criteria][1]* will not have *a*, it's plural. The singular will be criterion. 


  [1]: http://www.oxforddictionaries.com/definition/english/criterion?q=criteria