Renal physiology

11 ASCP MLS practice questions on Renal physiology, part of Urinalysis and Body Fluids. The first 11 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

   A 40-year-old man weighing 80 kg is having his creatinine clearance measured. His serum creatinine is 1.0 mg/dL [88 µmol/L]. He returns a 24-hour urine container holding 720 mL, and the urine creatinine is 100 mg/dL [8,840 µmol/L]. He mentions that he forgot to collect 'a couple of times' while at work. The laboratory's procedure states that adult men usually excrete about 20-25 mg of creatinine per kg of body weight per day [177-221 µmol/kg/day]. Which interpretation of these results is best?

   1. AValid result; low muscle mass lowers his creatinine
   2. BIncomplete collection; clearance is falsely low
   3. CTubular secretion falsely raising clearance
   4. DModerately reduced GFR, about 50 mL/min

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   Answer: B · Incomplete collection; clearance is falsely low

   Clearance = (U × V) / P = (100 mg/dL × 720 mL / 1,440 min) / 1.0 mg/dL = 50 mL/min. The container holds 100 mg/dL × 7.2 dL = 720 mg of creatinine, but an 80-kg man should excrete about 1,600 to 2,000 mg a day, so less than half the day's urine was collected. Because creatinine excretion is steady from day to day, total creatinine is the standard check of completeness for any 24-hour collection. With a normal serum creatinine and admitted missed voids, the low clearance is a collection error, not kidney disease. The test should be repeated with clear instructions to collect every void.

   • A: An 80-kg man with a normal serum creatinine is not expected to have low muscle mass, and low creatinine output would not lower the clearance itself.
   • C: Tubular secretion makes creatinine clearance overestimate GFR; it cannot explain a low clearance.
   • D: A clearance of 50 mL/min would mean reduced GFR, but his creatinine excretion is far below expected, which points to missed urine.

2. Question 2 · Easy

   Antidiuretic hormone (vasopressin) concentrates the urine by acting mainly on the:

   1. ACollecting ducts
   2. BGlomerular capillaries
   3. CAscending limbs of the loop of Henle
   4. DProximal convoluted tubules

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   Answer: A · Collecting ducts

   ADH from the posterior pituitary binds V2 receptors on collecting duct cells and inserts aquaporin-2 water channels into their luminal membrane. Water then moves out into the hypertonic medulla, so the urine becomes concentrated. Without ADH, or when the receptors do not respond, large volumes of dilute urine are passed (diabetes insipidus). ADH release rises with plasma osmolality and with low blood volume.

   • B: The glomerulus filters plasma; ADH does not act there.
   • C: The ascending limb stays impermeable to water, with or without ADH.
   • D: The proximal tubule reabsorbs water by osmosis whether or not ADH is present.

3. Question 3 · Hard

   A 31-year-old woman at 34 weeks of pregnancy has new hypertension. Her 24-hour urine volume is 2,400 mL with a protein of 35 mg/dL [0.35 g/L]. What is her 24-hour urine protein excretion?

   1. A840 mg/24 h [0.84 g/d]
   2. B8,400 mg/24 h [8.4 g/d]
   3. C350 mg/24 h [0.35 g/d]
   4. D84 mg/24 h [0.084 g/d]

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   Answer: A · 840 mg/24 h [0.84 g/d]

   First put the volume in the same unit as the concentration: 2,400 mL = 24 dL. Protein excretion = 35 mg/dL × 24 dL = 840 mg in 24 hours (0.35 g/L × 2.4 L = 0.84 g). This is well above the 300 mg/24 h that defines significant proteinuria in pregnancy, so it supports preeclampsia. Always check that the concentration and volume units match before multiplying.

   • B: 8,400 mg comes from dividing the volume by 10 instead of 100 to get decilitres.
   • C: 350 mg is the amount in one litre, without multiplying by the 2.4 L volume.
   • D: 84 mg comes from multiplying by 2.4 (litres) while keeping mg/dL.

4. Question 4 · Easy

   Effective renal plasma flow is estimated from the clearance of:

   1. Ap-Aminohippuric acid
   2. BEndogenous creatinine
   3. CUrea
   4. DInulin

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   Answer: A · p-Aminohippuric acid

   PAH is filtered and also secreted so efficiently by the proximal tubule that almost all of it is removed in one pass through the kidney. Its clearance therefore approximates the plasma flow to the kidney, about 600-700 mL/min in adults. Inulin, which is filtered but neither reabsorbed nor secreted, measures the GFR instead. PAH clearance is a research test rather than a routine one.

   • B: Creatinine clearance approximates the GFR.
   • C: Urea is partly reabsorbed, so its clearance is below the GFR.
   • D: Inulin clearance equals the GFR, not the plasma flow.

5. Question 5 · Easy

   A 24-hour urine collection is to start at 8:00 a.m. Which instruction is correct?

   1. ADiscard the first 8:00 a.m. void; collect the next day's 8:00 a.m. void
   2. BCollect the first 8:00 a.m. void; discard the next day's 8:00 a.m. void
   3. CCollect both the first and the next day's 8:00 a.m. voids
   4. DDiscard both the first and the next day's 8:00 a.m. voids

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   Answer: A · Discard the first 8:00 a.m. void; collect the next day's 8:00 a.m. void

   The bladder already holds urine made before the timed period, so the patient empties it at the start time and throws that urine away. Every later void goes into the container, ending with a final void at exactly the same time the next day. The sample then covers exactly 24 hours of urine production. The container is kept refrigerated or holds the preservative the test needs.

   • B: Keeping the first void adds urine made before the start, and discarding the last loses urine made during the period.
   • C: Keeping the first void adds urine made before the timed period began.
   • D: Discarding the final void loses urine made at the end of the period.

6. Question 6 · Medium

   Renal tubular cells make ammonia (NH3) from glutamine. How does this help the kidney excrete acid?

   1. ANH3 is secreted to raise the pH of the blood directly
   2. BNH3 blocks the secretion of H+ into the tubular lumen
   3. CNH3 combines with filtered bicarbonate to form urea
   4. DNH3 binds secreted H+ and is trapped in urine as NH4+

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7. Question 7 · Medium

   A substance's renal clearance is greater than the GFR. This shows that the substance is:

   1. ANeither reabsorbed nor secreted
   2. BReabsorbed by the tubules
   3. CBound to plasma proteins
   4. DSecreted by the tubules

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8. Question 8 · Medium

   Albumin (about 66 kDa) is near the size limit of the glomerular filter, yet very little crosses it. Besides size, the main reason is that:

   1. APodocytes pump filtered albumin back into blood
   2. BAlbumin and the filter are both negatively charged
   3. CAlbumin is too lipid-soluble to pass the filter
   4. DAlbumin is positively charged at blood pH

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9. Question 9 · Easy

   In a healthy kidney, nearly all filtered glucose is reabsorbed in which part of the nephron?

   1. ADescending limb of the loop of Henle
   2. BDistal convoluted tubule
   3. CProximal convoluted tubule
   4. DCollecting duct

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10. Question 10 · Hard

    A timed urine shows a flow of 4,320 mL per 24 hours and an osmolality of 96 mOsm/kg. Plasma osmolality is 288 mOsm/kg. What is the free water clearance?

    1. A-6.0 mL/min
    2. B-2.0 mL/min
    3. C+2.0 mL/min
    4. D+1.0 mL/min

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11. Question 11 · Medium

    Furosemide blocks the Na-K-2Cl cotransporter in the thick ascending limb. Why does this impair the kidney's ability to concentrate urine?

    1. AIt dissipates the medullary osmotic gradient
    2. BIt blocks ADH receptors in the collecting duct
    3. CIt makes the descending limb impermeable to water
    4. DIt sharply lowers the glomerular filtration rate

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