Membrane transport, action potential and body fluids

14 NEET-PG practice questions on Membrane transport, action potential and body fluids, part of Physiology. The first 14 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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