i think pulsus paradoxus is key here.I dont think in MI there would be pulsus paradoxus present...i will go for cardiac tamponade,coz kussmaul's sign excludes constrictive pericarditis,MI with hypotension is inferior wall MI in which there is vagal stimulation which also causes bradycardia and here there is tachycardia...regarding restrictive cardiomyopathy i dnt have much idea abt physical findings in that conditon...