This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'First heart sound is soft in all, except \nSelect and explain. \nA. Short PR interval, B. Ventricular septal defect, C. Mitral regurgitation, D. Calcified valve', 'role': 'user'}, {'content': 'Answer: A \nExplanation: S1 is loud in short PR interval\n"S1 is heard due to closure of AV valves"\n\nTHE INTENSITY OF S1\xa0 DEPENDS UPON -\n1)\xa0\xa0 Position of mitral leaflets at the onset of ventricular systole \n\nMitral leaflets at greater distance than normal - S1 loud (short PR intervar)\n\n\nMitral leaflets at a lesser distance than normal - S1 soft (long P-R intervals)\n\n2)\xa0\xa0 The rate of rise of left ventricular pressure \n\nLow rate of rise of pressure\nS1\xa0soft (VSD and MR)Q\nHigh rate of rise ofpressure\nS1 loud (ASQ)\n\n3)\xa0\xa0 Presence or absence of structural disease of mitral valve \n\nCalcification of mitral leafierQ →\xa0Soft S1\nDefective closure in MRQ →\xa0Soft S1\xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0\xa0\n\n4)\xa0\xa0 Amount of tissue, air or fluid between heart and stethoscope\n\nPericardial effusionQ\xa0→\xa0Soft S1\nObesityQ\xa0\xa0→ \xa0\xa0Soft S1 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0 \xa0\xa0\nEmphyseinaeQ\xa0→\xa0 Soft S1\n\nExplanation of the loud S1\xa0 in short PR interval\n\nPR interval denotes the time interval between atrial and ventricular contraction.\nPR interval occurs due to delay at the A.V. node. This delay allows maximal time for atrial contraction and protects the ventricle from fast stimulation. What would be the implication of early ventricular contraction on first heart sound ??\nIt has been already discussed that first heart sound occurs due to closure of the mitral valve at the start of ventricular systole or contraction.\nAt this time increased intraventricular pressure exceeds the atrial pressure and the A.V. valves close, producing first heart sound.\nKeep in mind one important fact that just before the start of the ventricular systole, the atrial contraction stops. (The atrial contraction occurs during late diastole, just before the start of ventricular systole).\nWith reduced PR interval the ventricle starts contracting, while the atria is still in a state of contraction.\nThus at the start of a ventricular contraction, the mitral valve leaflets are maximally separated because the atria is still contracting.\nThat means that ventricular systole will now push the mitral leaflet from a greater distance than usual.\nThis results in loud S (imagine the sound produced when a door is closed form a short distance or from a widely opened position. When the door is forced to shut from a large distance, obviously more sound will be produced).\nThe reverse happens in longer PR intervals -\nLarger PR interval means there is more time lag between atrial and ventricular contraction.\nThus when ventricular systole begins the mitral leaflets have already begun to close due to atrial relaxation.\nThus less force is applied to the mitral valve and its closing velocity is decreased resulting in soft S1', 'role': 'assistant'}]