In some cases, in a multiple choice question (Check All that Apply) -- an exclusive option ("I dont know" or "None of the Above") may be presented. In such cases you can mark those options as "Exclusive" options. This will enable the engine to validate if the respondent chooses that option - NO OTHER OPTION can be selected.
To set this up click on the Settings link for the Question
Here for the required option check mark the box under the Exclusive Option column.
Also make sure that Validation is enabled and Save the changes.