Wattle Pharmacy Prep

Renal physiology and fluid balance
Practice questions with explanations

Renal physiology and fluid balance belongs to Biomedical sciences, which makes up about 20% of the Overseas Pharmacist Readiness Assessment (OPRA®). Answer each question yourself before opening the explanation.

1. Which explains why potassium must be replaced during treatment of diabetic ketoacidosis?

  1. Total body potassium is depleted despite a normal or high serum level, and insulin drives potassium into cells
  2. Potassium is not affected
  3. Only sodium needs replacement
  4. Potassium rises during treatment
Show answer

A. Total body potassium is depleted despite a normal or high serum level, and insulin drives potassium into cells

Osmotic diuresis and acidosis shift potassium out of cells and out of the body, so the measured level conceals a large deficit. Starting insulin without replacement causes a rapid fall to dangerous levels, which is why potassium is checked before and frequently during treatment and added to fluids once it is not high.

2. Where does the majority of filtered sodium reabsorption occur in the nephron?

  1. The proximal convoluted tubule
  2. The glomerulus
  3. The distal convoluted tubule
  4. The collecting duct
Show answer

A. The proximal convoluted tubule

About two thirds of filtered sodium, with water following isosmotically, is reclaimed in the proximal tubule, along with glucose, amino acids and bicarbonate. The thick ascending limb takes another quarter, the distal tubule around five per cent and the collecting duct a few per cent under aldosterone control. The size of each segment's contribution predicts diuretic potency, which is why loop diuretics are the most powerful.

3. Which drugs are most often implicated in drug-induced acute kidney injury?

  1. NSAIDs, ACE inhibitors or sartans, diuretics, aminoglycosides and contrast media
  2. Inhaled corticosteroids and beta-2 agonists
  3. Proton pump inhibitors, exclusively through direct tubular toxicity
  4. Topical emollients
Show answer

A. NSAIDs, ACE inhibitors or sartans, diuretics, aminoglycosides and contrast media

The common mechanisms are haemodynamic, where NSAIDs, renin-angiotensin blockers and diuretics combine to strip glomerular perfusion, and direct tubular toxicity from aminoglycosides and contrast. Proton pump inhibitors cause acute interstitial nephritis, an immune-mediated rather than toxic mechanism, and are a genuinely common cause worth remembering. Sick day guidance to withhold certain medicines during dehydrating illness follows from this.

These are 3 of the 40 questions on renal physiology and fluid balance in the app.

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