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': 'A patient following head injury was admitted in intensive care ward with signs of raised intracranial pressure. He was put on a ventilator and started on intravenous fluids and diuretics. Twenty-four hours later his urine output was 3.5 litres, serum sodium 156 mEq/I and serum osmolarity of 316 mOsm/kg. The most likely diagnosis based on these parameters is - \nSelect and explain. \nA. High output due to diuretics, B. Diabetes insipidus, C. Too much infusion of normal saline, D. Cerebral salt retaining syndrome', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Laboratory abnormalities seen in this patient\n\nIncreased serum sodium\xa0\xa0\xa0\xa0\xa0\xa0\xa0→\xa0 (N-135-145 meq/dL)\nIncreased serum osmolarity \xa0→\xa0\xa0 (N-275-293 mosm/kg)\nPolyuria\xa0 \xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0→\xa0\xa0 (Urine output > 3L/day sometimes assumed to be 70 kg, the\n\ncriteria forpolyuria is > 3.5L/day)\nUsually when the diagnosis of polyuria is Established the next step is to assess urine osmolality\nUrine osmolarity\n> 300 mosmol/L\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Solute diuresis due to diabetes Mellitus or\xa0Diuretics (Osmotic diuretics and loop diuretics)\n< 300 mosm/L\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0 Water diuretis (Now, it should be further evaluated to determine which type of diabetes insipidus is present)\n\nIn the question there is no information regarding the urine osmolality and specific gravity\nThe other clues given in the question -\n\n-H/O head injury\n-Raised intracranial tension\n-Patient is on diuretic (Usually is given to reduce pressure) \n-L V. fluid administration intracranial\n\xa0\n\nThe history and the laboratory abnormalities suggests either diabetes insipidus or diuresis due to osmotic diuretics.\nThe features of Hypernatrenzia, Increased osmolality and increased urine output indicates diuresis due to mannitol.\nPts with head injury and increased intracranial pressure requires mannitol adminstration (to reduce intracranial pressure).\nMannitol is an osmotic diuretic, it is freely filtered at glomerulus and undergoes limited reabsorption.\nPresence of mannitol in the tubular fluid reduces the corticomedullary gradiant and retains water isoosmatically in the proximal tubular fluid.\nSo, the primary function of mannitol is to increase urinary volume.\nThe loss of water in the urine can cause Hypernatremia and increase in serum osmolarity.\n\n"Whenever mannitol is administered in cases of head trauma to reduce the intracranial tension, serum osmolarity should always be monitored (it should never be allowed to go beyond 320 mosmol/kg.)"\nDiabetes insipidus\n\nHypernatremia and increase in Serum osmolarity can also occur in Diabetes insipidus (due to increased water loss).\nPresence of head injury too, suggests Diabetes insipidus\nBut, this is not a case of diabetes insipidus\n\nTo establish the diagnosis of diabetes insipidus urinary osmolarity and specific gravity is needed\n- urine osmolarity < 300 mosmol/kg and\n- specific gravity < 1.02\nUrine output in cases of diabetes insipidus is generally in the range of 6-7 L/day.\n(Here, the urine output is 3.5 L/day which is just adequate to meet the criteria of polyuria)\nRemember,\nThe pt is on a diruretic i.e., mannitol\n\nMannitol will prevent reabsorption of water in\xa0\xa0\xa0\xa0\xa0\xa0\xa0 \xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0→\xa0\xa0\xa0\xa0\xa0\xa0 \xa0Proximal tubule\n. Diabetes insipidus will prevent reabsorption water in\xa0 \xa0→\xa0\xa0\xa0\xa0\xa0 Collecting duct of \n\n\n\xa0\nSo the presence of diabetes insipidus in patients on diuretic will lead to huge loss of water through urine which will certainly be much more than 3.5 L/day\nNote :\nThe patients is on ventilator\n\nSignificant loss of water also occurs through ventilation by evaporation. This may also contribute to hypernatremia and increase in serum osmolarity.\n\nToo much infusion of normal saline\n\nToo much infusion of normal saline will never increase sodium and serum osmolarity in physiologically normal patient\nHypernatremia can be caused by administration of hypertonic saline and NaHCO3.', 'role': 'assistant'}]