A patient shows almost no interest in activities he previously enjoyed, rarely initiating anything on his own, and appears outwardly indifferent even when told genuinely significant news about his own health. Before revealing the answer: is this constriction of affect?

No, this is more consistent with apathy than with constriction of affect. Constriction of affect specifically concerns a narrowed range of emotional display with interest otherwise intact; here, the pervasive lack of interest, reduced initiation of activity, and indifference even to significant personal news is the defining pattern of apathy rather than a narrowed emotional range.