Problems with exam questions are often picked up after the exam. Question correction differs from Question exclusion in that a question can only be corrected if the wrong answer has originally been set as correct.
The Frequency & Discrimination (U-L) Analysis report should be opened to find a poorly performing question. Say, for example that the question "What is the capital of the UK?" has the answersLondon, Edinburgh and Cardiff. 80% choose London, which is the correct answer, but the setter has mistakenly put Cardiff. The report will show a disproportionate number of students picking a 'wrong' answer.
Once the offending question has been located, the examiner needs to close the report, edit the question and save it. In the case of a locked question (as it almost certainly will be) you can change the answer but very few other details, such as increasing or decreasing the marks. You also only have the Limited Save option available.
Once you have done this, opening the report will show the correct answer in bold along with the corresponding percentages. Rogo will re-mark the question for you.