Membrane transport, action potential and body fluids

28 NEET-PG practice questions on Membrane transport, action potential and body fluids, part of Physiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   At a PHC in Odisha during a cholera outbreak, a 25-year-old woman with profuse watery diarrhoea is treated with WHO low-osmolarity ORS. The medical officer explains the physiology to interns: 1. Sodium and glucose enter the enterocyte together through SGLT1 on the apical membrane. 2. Cholera toxin works by inactivating SGLT1. 3. The total osmolarity of WHO low-osmolarity ORS is 245 mOsm/L. 4. Sodium leaves the enterocyte across the basolateral membrane through the Na+-K+ ATPase. Which statements are correct?

   1. A1, 3 and 4 only
   2. B1 and 2 only
   3. C3 and 4 only
   4. D1, 2 and 4 only

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   Answer: A · 1, 3 and 4 only

   Statements 1, 3 and 4 are correct. SGLT1 is a secondary active cotransporter that uses the sodium gradient, maintained by the basolateral Na+-K+ ATPase, to carry glucose and sodium into the enterocyte together, and water follows the absorbed solute. Cholera toxin raises cAMP by ADP-ribosylating the Gs protein, which opens CFTR and drives chloride secretion, but it leaves SGLT1 working, so statement 2 is wrong and ORS can still replace losses. The reduced-osmolarity formula (sodium 75 and glucose 75 mmol/L, total 245 mOsm/L) lowers stool output and vomiting compared with the older formula.

   • B: Includes statement 2 and omits statements 3 and 4, both true.
   • C: Omits statement 1, the SGLT1 cotransport that ORS relies on.
   • D: Includes statement 2; cholera toxin acts through cAMP and spares SGLT1.

2. Question 2 · Easy

   In a research study of body water in healthy medical students, the team wants to measure each volunteer's extracellular fluid volume by the dilution method. Which marker suits this purpose?

   1. AEvans blue dye
   2. BInulin
   3. CAntipyrine
   4. DDeuterium oxide

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   Answer: B · Inulin

   A marker for extracellular fluid must cross capillary walls freely but stay out of cells. Inulin, a polysaccharide, does exactly this, as do mannitol, sucrose and radioactive sodium or thiosulphate. Volume is the amount injected (minus any excreted) divided by the plasma concentration after mixing. Intracellular fluid cannot be measured directly; it is total body water minus extracellular fluid.

   • A: Evans blue binds albumin and stays in the vessels, so it measures plasma volume.
   • C: Antipyrine is lipid-soluble, enters cells and measures total body water.
   • D: Deuterium oxide (heavy water) enters cells and measures total body water.

3. Question 3 · Medium

   In the physiology practical class, first-year MBBS students record a resting potential of -70 mV from a nerve fibre. Consider four statements: 1. The Na+-K+ pump contributes a few millivolts directly, because it moves 3 Na+ out for every 2 K+ in. 2. The potential lies close to E_K because resting permeability to K+ is far higher than to Na+. 3. Halving the extracellular Na+ concentration would make the resting potential markedly less negative. 4. The Goldman-Hodgkin-Katz equation predicts it from the concentrations of K+, Na+ and Cl- and the membrane's permeability to each. Which statements are correct?

   1. A1, 2 and 4 only
   2. B2, 3 and 4 only
   3. C1 and 2 only
   4. D2 and 4 only

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   Answer: A · 1, 2 and 4 only

   Statements 1, 2 and 4 are correct. The pump is electrogenic and adds a small negativity directly, but its main role is to maintain the concentration gradients. At rest, K+ leak channels make the membrane many times more permeable to K+ than to Na+, so the Goldman equation, which weighs each ion's concentrations by its permeability, gives a value close to E_K but slightly less negative because of the small Na+ leak. Statement 3 is wrong: because resting Na+ permeability is low, a change in extracellular Na+ alters the resting potential very little, although it reduces the height of the action potential.

   • B: Includes statement 3; extracellular Na+ has little effect at rest, and omits statement 1.
   • C: Omits statement 4; the Goldman equation is exactly how the potential is predicted.
   • D: Omits statement 1; the pump does make a small direct contribution.

4. Question 4 · Medium

   A 30-year-old construction worker in Barmer, Rajasthan, works through a hot May afternoon, sweating heavily and drinking little water. By evening he is thirsty, with a dry tongue; serum Na+ is 152 mmol/L (reference 135-145). Consider four statements comparing his body fluids with the morning: 1. Extracellular fluid volume has fallen. 2. Intracellular fluid volume has fallen. 3. Plasma osmolality has risen. 4. Osmolality inside his cells has also risen. Which statements are correct?

   1. A1, 2, 3 and 4
   2. B1, 2 and 3 only
   3. C1, 3 and 4 only
   4. D1 and 3 only

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   Answer: A · 1, 2, 3 and 4

   All four statements are correct. Sweat is hypotonic, so he loses more water than salt from the extracellular fluid, whose volume falls and whose osmolality rises. Water then moves out of the cells by osmosis until intracellular osmolality equals the new, higher extracellular osmolality, so both compartments shrink and both become more concentrated. This is the pattern of hypertonic dehydration on a Darrow-Yannet diagram; treatment is water replacement, with salt as well if losses are large.

   • B: Omits statement 4; cell osmolality rises to match the extracellular fluid.
   • C: Omits the shrinking of the cells, which must occur when cell osmolality rises.
   • D: Omits the fall in cell volume and the rise in cell osmolality, which follow from osmotic water shift.

5. Question 5 · Medium

   A 20-year-old student who is anxious about her viva breathes fast and deep for several minutes, then develops tingling of the lips and fingers and carpopedal spasm. Her total serum calcium is normal. How does her overbreathing make her nerves hyperexcitable?

   1. AHypocapnia slows the sodium-potassium pump in the axon
   2. BHypocapnia increases acetylcholine release at the junction
   3. CAlkalosis lowers ionised calcium, so sodium channels open more easily
   4. DAlkalosis raises potassium, so the membrane depolarises

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   Answer: C · Alkalosis lowers ionised calcium, so sodium channels open more easily

   Hyperventilation washes out CO2 and causes respiratory alkalosis. At a higher pH, albumin binds more calcium, so the ionised fraction falls even though total calcium is normal. Extracellular calcium normally stabilises voltage-gated sodium channels; when it falls, they open with a smaller depolarisation, so nerves fire spontaneously, producing tingling and tetany. Reassurance and slow breathing correct it, but true hypocalcaemia should be excluded if spasm recurs.

   • A: The Na+-K+ pump is not the reason for the hyperexcitability of tetany.
   • B: Low calcium actually reduces acetylcholine release, because calcium entry triggers release.
   • D: Alkalosis shifts potassium into cells and tends to lower, not raise, serum potassium.

6. Question 6 · Hard

   A 66-year-old man with acute kidney injury has a serum K+ of 8.0 mmol/L. Assume an intracellular K+ concentration of 140 mmol/L and use the simplified Nernst equation at body temperature, E = -61.5 x log10([K+] inside / [K+] outside) mV (log10 17.5 = 1.24; log10 35 = 1.54). What is his potassium equilibrium potential?

   1. AAbout +76 mV
   2. BAbout -38 mV
   3. CAbout -76 mV
   4. DAbout -95 mV

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7. Question 7 · Medium

   A medical student asks why a nerve cannot fire a second spike in the first millisecond after an action potential, however strong the second stimulus is. What is the reason for this absolute refractory period?

   1. AThe Na+-K+ pump is still restoring ion gradients
   2. BPotassium channels have not yet opened fully
   3. CInactivation gates of the sodium channels are closed
   4. DExtracellular sodium has been used up locally

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8. Question 8 · Medium

   A 19-year-old man has had attacks of limb weakness since childhood, each lasting under an hour, usually while resting after exercise or after skipping a meal. During an attack serum K+ is 6.1 mmol/L, and EMG between attacks shows myotonic discharges. His father had similar episodes. A mutation in which channel is most likely?

   1. ARyanodine receptor channel (RYR1)
   2. BL-type calcium channel (CACNA1S)
   3. CSkeletal muscle sodium channel (SCN4A)
   4. DSkeletal muscle chloride channel (CLCN1)

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9. Question 9 · Medium

   A 58-year-old man on maintenance haemodialysis misses two sessions and comes to casualty with weakness of his legs. Serum K+ is 7.4 mmol/L (reference 3.5-5.0) and the ECG shows tall, peaked T waves. What has happened to the resting membrane potential of his muscle cells?

   1. AIt is unchanged, as the pump compensates
   2. BIt is unchanged, as it depends on sodium
   3. CIt has become more negative (hyperpolarised)
   4. DIt has become less negative (depolarised)

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10. Question 10 · Medium

    A 16-year-old boy from Kottayam finds that his hands stiffen when he grips something firmly, so he cannot let go of a door handle quickly; the stiffness eases after he grips and releases a few times. Percussion of the thenar muscles causes a sustained dimple, and EMG shows myotonic discharges. His muscles look well developed and serum K+ is normal. Which channel defect is most likely?

    1. AAntibodies blocking the acetylcholine receptor
    2. BReduced chloride conductance of the muscle membrane
    3. CExcess sodium current through inactivation-defective channels
    4. DLoss of calcium release from the sarcoplasmic reticulum

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11. Question 11 · Hard

    A 70-kg man is kept nil by mouth after surgery and receives 1 litre of 5% dextrose over four hours. Take total body water as 60% of body weight, intracellular fluid as two-thirds of body water, and plasma as one-quarter of extracellular fluid. Once the glucose has been metabolised, roughly how much of the infused water remains in his plasma?

    1. AAbout 330 mL
    2. BAbout 80 mL
    3. CAbout 1,000 mL
    4. DAbout 250 mL

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12. Question 12 · Easy

    A 65-year-old woman with heart failure and fast atrial fibrillation is started on digoxin in the medicine ward. How does digoxin increase the force of contraction of her ventricular muscle?

    1. AStimulates beta-1 receptors on the myocytes
    2. BInhibits Na+-K+ ATPase, so cell calcium rises
    3. COpens L-type calcium channels directly
    4. DInhibits phosphodiesterase 3 in the myocytes

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13. Question 13 · Easy

    A 6-year-old boy in a Chennai paediatric OPD has recurrent chest infections, bulky greasy stools and poor weight gain. His sweat chloride is 78 mmol/L (normal below 30 mmol/L). The defective protein in his disease is best described as which kind of membrane protein?

    1. AA voltage-gated sodium channel
    2. BA sodium-potassium exchange pump
    3. CA ligand-gated cation channel
    4. DA cAMP-regulated chloride channel

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14. Question 14 · Medium

    A 4-year-old boy from a village in Bihar has had puffiness of the face and swelling of the legs and abdomen for three weeks. Urine protein is 4+, serum albumin is 1.6 g/dL, blood pressure is normal and the urine shows no red cells. Which change in Starling forces is mainly responsible for his oedema?

    1. AObstruction of lymphatic drainage
    2. BRise in capillary hydrostatic pressure
    3. CRise in capillary permeability to protein
    4. DFall in plasma colloid osmotic pressure

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15. Question 15 · Medium

    Plasma sodium is 140 mmol/L and the sodium concentration inside a skeletal muscle fibre is 14 mmol/L. At body temperature the Nernst equation is E = 61 log (Co/Ci) for a monovalent cation. A physiology tutor asks the house surgeons to work out the sodium equilibrium potential. What value do they get?

    1. AAbout -61 mV
    2. BAbout +61 mV
    3. CAbout +122 mV
    4. DAbout -90 mV

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16. Question 16 · Easy

    Which indicator is used to measure total body water by the dilution method?

    1. ADeuterium oxide
    2. BEvans blue dye
    3. CInulin
    4. DRadio-iodinated albumin

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17. Question 17 · Hard

    In the medical ICU, a young man with type 1 diabetes who stopped his insulin during a week of fever and vomiting is admitted drowsy, with deep sighing breaths and dry mucosa. Arterial pH is 7.08 and urine ketones are strongly positive. He has blood glucose 900 mg/dL and measured serum sodium 128 mmol/L (normal 135-145). The resident wants his true sodium status before choosing fluids. Using a correction of 1.6 mmol/L of sodium for every 100 mg/dL of glucose above 100 mg/dL, what is his corrected sodium, and what does it show?

    1. AAbout 141 mmol/L; water has shifted out of cells, not true hyponatraemia
    2. BAbout 152 mmol/L; hypernatraemia from free-water loss
    3. CAbout 128 mmol/L; true hypotonic hyponatraemia needing hypertonic saline
    4. DAbout 134 mmol/L; mild true hyponatraemia from renal salt loss

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18. Question 18 · Hard

    A man on twice-weekly haemodialysis for diabetic kidney disease misses three sessions during a family wedding in his village and eats plenty of bananas, dried fruit and coconut water. His wife brings him to casualty when he cannot get up from his chair. His serum potassium has risen to 7.8 mmol/L (normal 3.5-5.0). Early in the rise he had paraesthesiae and twitching; now he has flaccid weakness of all four limbs with absent reflexes. His resting membrane potential has moved from -90 mV to about -60 mV. Why has the early hyperexcitability given way to paralysis?

    1. ASustained depolarisation keeps sodium channel inactivation gates closed
    2. BHigh potassium opens extra potassium channels and hyperpolarises the fibre
    3. CPotassium competes with calcium at the troponin binding site
    4. DThe membrane moves nearer threshold, so action potentials fire more easily

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19. Question 19 · Medium

    Consider these statements about the Na+-K+ ATPase: 1. Each cycle moves three Na+ out and two K+ into the cell. 2. It adds a small negative contribution to the resting membrane potential. 3. Glucose uptake by SGLT1 is another example of primary active transport. 4. Cardiac glycosides inhibit it by binding its alpha subunit on the outer surface. Which statements are correct?

    1. A2, 3 and 4 only
    2. B1, 2 and 4 only
    3. C1, 2, 3 and 4
    4. D1 and 3 only

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20. Question 20 · Medium

    Serum sodium is 112 mmol/L (normal 135-145) in a woman with schizophrenia who has been drinking water compulsively for a week; she is alert, with mild nausea. On the ward her sodium is raised with hypertonic saline to 130 mmol/L within 24 hours. Four days later she develops dysarthria, dysphagia and quadriparesis. Which mechanism best explains this new problem?

    1. AHypertonic saline directly inhibits the neuronal Na+-K+ ATPase
    2. BPlasma sodium crosses the blood-brain barrier and depolarises neurons
    3. CWater moves into brain cells and causes cerebral oedema
    4. DBrain cells that shed osmolytes shrink as plasma tonicity rises fast

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21. Question 21 · Medium

    Lab report from the casualty, 1 a.m.: sodium 136 mmol/L, glucose 90 mg/dL, blood urea nitrogen 14 mg/dL, measured serum osmolality 335 mOsm/kg (normal 275-295). A labourer is drowsy with blurred vision after drinking illicit liquor the night before. Using calculated osmolality = 2 Na + glucose/18 + BUN/2.8, what is the osmolar gap?

    1. AAbout 35 mOsm/kg
    2. BAbout 85 mOsm/kg
    3. CAbout 53 mOsm/kg
    4. DAbout 13 mOsm/kg

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22. Question 22 · Easy

    During a cholera outbreak in a coastal ward, at a camp run by the district health team, children with profuse watery diarrhoea are given WHO low-osmolarity ORS by mouth. Their stool losses stay high, yet they rehydrate well. Which transport process allows the ORS to work even though cholera toxin is still acting?

    1. AChloride secretion through the CFTR channel
    2. BPrimary active transport by Na+-K+ ATPase
    3. CFacilitated diffusion of glucose by GLUT5
    4. DSodium-glucose cotransport by SGLT1

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23. Question 23 · Medium

    Tremor, heat intolerance, weight loss of 5 kg and palpitations for two months, and then, after a carbohydrate-rich dinner, a young man wakes unable to move his limbs. His speech and breathing are normal. Serum potassium is 2.1 mmol/L (normal 3.5-5.0) and free T4 is high. Which mechanism mainly explains his low potassium?

    1. AA mutant skeletal muscle sodium channel leaks potassium out of cells
    2. BThyroid hormone increases renal potassium excretion through aldosterone
    3. CThyroid hormone and insulin increase Na+-K+ ATPase activity, driving K+ into cells
    4. DDiarrhoea from thyrotoxicosis causes heavy faecal potassium loss

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24. Question 24 · Hard

    In the casualty at midnight, a man on haemodialysis who missed two sessions has a serum potassium of 7.8 mmol/L (3.5-5.0) and tall, peaked T waves with a widening QRS. The duty doctor pushes calcium gluconate 10% over five minutes, and the QRS narrows within minutes, though the repeat potassium is unchanged. Insulin-dextrose has not been started yet. The panel shows the resting potential and threshold of a ventricular cell in five states. Which panel fits this cell ten minutes after the calcium?

    

    −+Fit to screenClose

    

    1. APanel S
    2. BPanel Q
    3. CPanel P
    4. DPanel R

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25. Question 25 · Medium

    In the medical ward, a house officer plots one patient's body-fluid compartments on a volume-osmolarity diagram before writing the fluid orders. The normal state is dashed and the patient's state is solid; intracellular and extracellular fluid lie on either side of the central line. The plot is shown. Which condition does it fit?

    

    −+Fit to screenClose

    

    1. ACentral diabetes insipidus with poor water intake
    2. BPrimary adrenal insufficiency
    3. CSyndrome of inappropriate ADH
    4. DIsotonic fluid loss in severe cholera

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