Physiology — Body Fluids, Sodium and Potassium, NMC MBBS licence examination syllabus (Nepal Medical Council).
NMC-style practice questions written for this site. They are not past questions, and no past paper has been reproduced.
Why is a low serum sodium usually described as a water problem rather than a salt deficiency?
Answer: Because the measurement is a concentration — sodium relative to water — rather than a measure of total body sodium. Retaining water lowers the concentration even when total body sodium is normal or raised, which is why the useful first step is assessing the patient's fluid state rather than giving salt.
An elderly patient has been confused for several days and is found to have a low sodium. Why must correction be gradual?
Answer: Because the brain adapts to a sodium that has fallen slowly, and raising it quickly causes irreversible neurological injury. The risk is greatest precisely in long-standing disturbance. Correction should be slow and monitored, with senior involvement early.
A routine sample from a well outpatient shows a high potassium. The venepuncture was difficult. The ECG is normal. What is the appropriate response?
Answer: Repeat the sample before treating. Potassium leaks from damaged red cells, so a haemolysed specimen raises the measured value artificially, and a difficult venepuncture makes that likely. Treatment should not wait only if the patient is unwell or the ECG is abnormal.
Why does an ECG matter more than the potassium result itself in a patient with hyperkalaemia?
Answer: Because potassium disturbance is dangerous through its effect on membrane excitability and cardiac conduction, and that effect varies between patients. Two patients with the same concentration are not in the same danger; the ECG shows which one needs immediate treatment.
A patient with liver failure has gross peripheral oedema and a low blood pressure. Why is a diuretic not automatically the right response?
Answer: Because oedema and an under-filled circulation can coexist. Fluid has accumulated in the tissues while the circulating volume remains inadequate, so removing fluid may worsen perfusion. This patient needs careful assessment of the circulating volume rather than a reflex response to the oedema.
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