Accessing parent properties inside relationship query

An example:

$passing_students = App\Exam::whereHas('students', function ($query) {
    $query->where('mark', '>=', $exam->pass_mark);
})->get();

I am interested in fetching all exams with students who passed however in the relationship query function I'm not sure how to access the parent model's properties such that I can complete the comparison. What should be in place of $exam->pass_mark?

Note that I'm looking for a solution that is done within the single query builder as I'm aware that this can be easily done in a separate foreach loop.

via Chebli Mohamed