Right or left isomerism with an unpredictable situs arrangement defines which term?

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Situs ambiguous, or heterotaxy, is the situation where the arrangement of the heart and visceral organs cannot be clearly classified as normal (situs solitus) or complete mirror image (situs inversus) and there is unpredictable, bilateral isomerism. This term specifically captures right- or left-sided isomerism with an unclear overall situs.

This is why it’s the best answer: when the left-right pattern is not determinable and the situs arrangement is mixed or ambiguous, the description used is situs ambiguous. Dextrocardia describes the heart being positioned on the right side, but the overall visceral arrangement isn’t, by itself, defined as unpredictable situs; mesocardia refers to the heart being central rather than laterally displaced; and the term you provided isn’t a recognized descriptor in this context.