What defines a pathologic Q wave on ECG?

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A pathologic Q wave signals scar from a prior transmural infarction and is defined by two features in a given lead: it must be wide and relatively deep compared to the following R wave. Specifically, a Q wave that lasts at least 0.04 seconds (one small box on the standard ECG) and is at least one-quarter the amplitude of the next R wave in that lead meets the usual criterion. This combination helps distinguish a true pathologic Q wave from normal, small Q waves that can occur in healthy individuals or due to normal variants. So the defining idea is that a pathologic Q wave is both sufficiently wide and sufficiently deep relative to the ensuing R wave. The other options fail because they either use an inappropriate duration (too short), an incorrect depth ratio (too large), or they apply a rule to a single lead rather than recognizing the pattern across leads, which is not how pathologic Q waves are identified.