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BI9.1-3 | Minerals, electrolytes, Water and Acid base balance — Gate Quiz
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Which cation is the major intracellular cation in human cells?
Correct! Potassium (K+) is the major intracellular cation (approximately 140 mEq/L intracellularly vs. 4-5 mEq/L extracellularly).
ICF (intracellular fluid): K+ (major cation), phosphate (major anion), proteins. ECF (extracellular fluid): Na+ (major cation), Cl- and HCO3- (major anions). The Na+/K+ ATPase pump maintains this gradient, essential for resting membrane potential and nerve/muscle function.
Incorrect. K+ is the major intracellular cation; Na+ is the major extracellular cation.
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Serum osmolality is approximately 285-295 mOsm/kg and is primarily determined by which electrolyte?
Correct! Sodium and its accompanying anions (Cl-, HCO3-) account for approximately 90% of serum osmolality. The osmolality formula is: 2[Na+] + [glucose]/18 + [BUN]/2.8.
Calculated osmolality = 2[Na+] + [Glucose mg/dL]/18 + [BUN mg/dL]/2.8. Osmol gap = measured - calculated (normal < 10). Elevated osmol gap indicates presence of unmeasured osmoles (e.g., ethanol, methanol, mannitol).
Incorrect. Sodium is the primary determinant of serum osmolality.
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A 70-year-old patient on diuretics develops serum Na+ of 118 mEq/L (severe hyponatraemia) with confusion and seizures. Which complication is most feared from overly rapid correction of serum sodium?
Correct! Rapid correction of chronic hyponatraemia causes osmotic demyelination syndrome (central pontine myelinolysis), where rapid osmotic shifts strip water from brain cells, causing demyelination and permanent neurological damage.
Safe correction rate: no more than 8-10 mEq/L per 24 hours (some guidelines 10-12 mEq/L/day). Correction should be slower in chronic hyponatraemia (>48 hours duration). Acute severe symptomatic hyponatraemia: 3% saline to raise Na+ by 1-2 mEq/L/hour until symptoms resolve, then slow correction.
Incorrect. Osmotic demyelination syndrome is the feared complication of rapid correction of hyponatraemia.
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Iron absorption from the gut is maximized when iron is in which form, and which vitamin enhances this?
Correct! Iron is absorbed as Fe2+ (ferrous) by the DMT1 transporter. Vitamin C (ascorbic acid) reduces Fe3+ to Fe2+, enhancing absorption. Gastric acid also facilitates this reduction.
Iron absorption factors: INCREASES absorption: Fe2+, Vitamin C, acidic pH, haem iron (animal sources), iron deficiency state. DECREASES absorption: Fe3+, alkaline pH, phytates (chapati, rice), tannins (tea, coffee), calcium, antacids. Transferrin transports iron in blood; ferritin stores it intracellularly.
Incorrect. Fe2+ (ferrous form) is absorbed by DMT1; Vitamin C reduces Fe3+ to Fe2+.
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A patient with hypoparathyroidism has serum Ca2+ of 1.8 mmol/L (normal 2.2-2.6). The patient develops carpopedal spasm (Trousseau sign). This occurs because low calcium causes:
Correct! Low calcium (hypocalcaemia) increases membrane excitability by reducing the threshold for nerve depolarisation, causing spontaneous firing (tetany, carpopedal spasm, Chvostek sign, laryngospasm).
Ca2+ regulation: PTH increases serum Ca2+ (osteoclast activation, renal reabsorption, 1-alpha-hydroxylase activation). Calcitonin decreases serum Ca2+. Calcitriol (1,25-dihydroxy-D3) increases intestinal Ca2+ absorption. Hypoparathyroidism causes hypocalcaemia with hyperphosphataemia.
Incorrect. Hypocalcaemia increases membrane excitability, causing spontaneous depolarisation and tetany.
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A patient with uncontrolled Type 1 diabetes has: Na+ 138, Cl- 98, HCO3- 10 mEq/L. The anion gap is:
Correct! Anion gap = Na+ - (Cl- + HCO3-) = 138 - (98 + 10) = 138 - 108 = 30 mEq/L. Normal is 8-12 mEq/L. This elevated anion gap indicates accumulation of unmeasured anions (ketoacids in DKA).
High anion gap metabolic acidosis causes (MUDPILES): Methanol, Uraemia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. Normal anion gap (hyperchloraemic) acidosis: diarrhoea, RTA, ammonium chloride ingestion.
Incorrect. Anion gap = Na+ - (Cl- + HCO3-) = 138 - 108 = 30 mEq/L — significantly elevated.
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A patient with COPD has pH 7.28, pCO2 65 mmHg, HCO3- 28 mEq/L. This is best described as:
Correct! pH is low (acidosis), pCO2 is elevated (respiratory cause), HCO3- is elevated (renal compensation). This is respiratory acidosis with metabolic (renal) compensation.
Acid-base analysis: (1) pH below 7.35 = acidosis, above 7.45 = alkalosis. (2) Check pCO2: elevated in respiratory acidosis. (3) Check HCO3-: if elevated in acidosis = metabolic compensation. COPD patients retain CO2 chronically; kidneys retain HCO3- to compensate. Expected HCO3- compensation = 3.5 x (pCO2 - 40)/10 for chronic.
Incorrect. Elevated pCO2 with low pH = respiratory acidosis; elevated HCO3- = renal compensation.
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A patient has been vomiting repeatedly for 3 days due to pyloric stenosis. ABG shows pH 7.52, pCO2 48 mmHg, HCO3- 38 mEq/L. This is:
Correct! pH is high (alkalosis), HCO3- is elevated (metabolic cause), pCO2 is slightly elevated (respiratory compensation by hypoventilation). This is metabolic alkalosis with respiratory compensation, caused by HCl loss from vomiting.
Vomiting causes loss of HCl from the stomach, increasing serum HCO3- (metabolic alkalosis). The kidneys normally excrete excess HCO3-, but concomitant hypovolaemia causes renal Na+/HCO3- reabsorption to maintain volume (paradoxical aciduria). Treatment: IV saline + KCl.
Incorrect. High pH + high HCO3- = metabolic alkalosis. Elevated pCO2 = respiratory compensation.
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Zinc is an essential trace element that is a structural component or cofactor for many enzymes and transcription factors. Zinc deficiency characteristically causes:
Correct! Zinc deficiency causes acrodermatitis enteropathica (dermatitis around body orifices), hypogonadism, growth retardation, poor wound healing, impaired immune function, and anosmia.
Zinc functions: cofactor for >300 enzymes (carbonic anhydrase, carboxypeptidase, alcohol dehydrogenase), structural role in zinc-finger transcription factors, immune function, wound healing, taste and smell. Sources: meat, seafood (especially oysters), nuts. Zinc deficiency common in vegans and malabsorption states.
Incorrect. Zinc deficiency causes acrodermatitis enteropathica, hypogonadism, and growth failure.
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In a 70 kg adult male, total body water (TBW) is approximately 42 L (60% of body weight). What proportion of TBW is intracellular fluid (ICF)?
Correct! ICF is approximately two-thirds (2/3) of total body water = about 28 L. ECF is one-third (1/3) = about 14 L. ECF is further divided into plasma (25%) and interstitial fluid (75%).
Body fluid distribution: TBW = 60% body weight (males; 50% females). ICF = 2/3 TBW (approximately 28 L). ECF = 1/3 TBW (approximately 14 L). ECF subdivisions: Plasma = 25% ECF (approximately 3.5 L); Interstitial fluid = 75% ECF (approximately 10.5 L). Transcellular fluid (CSF, pleural, peritoneal) = small additional compartment.
Incorrect. ICF = 2/3 of TBW; ECF = 1/3 of TBW.
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