Give me 10 mcq with solutions for kidney function test

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10 MCQs on Kidney Function Tests

Q1. What is the gold standard method for measuring GFR?

A) Serum creatinine measurement B) 24-hour urine creatinine clearance C) Inulin clearance D) Cystatin C measurement

Answer & Explanation

✅ C) Inulin clearance

Inulin is the ideal GFR marker because it is freely filtered at the glomerulus and is neither reabsorbed nor secreted by the tubules. Inulin clearance (urinary clearance during continuous IV infusion) has long been regarded as the gold standard measure of GFR, though it is impractical for routine clinical use.

— Tietz Textbook of Laboratory Medicine, 7e; National Kidney Foundation Primer on Kidney Diseases, 8e; Comprehensive Clinical Nephrology, 7e

Q2. A 45-year-old man has a BUN of 56 mg/dL and serum creatinine of 1.1 mg/dL. The BUN/creatinine ratio is approximately 51:1. Which condition best explains this pattern?

A) Acute tubular necrosis B) Chronic renal failure C) Prerenal azotemia D) Postrenal obstruction

Answer & Explanation

✅ C) Prerenal azotemia

The BUN/creatinine ratio is normally 10:1 to 20:1. In prerenal azotemia (due to reduced renal perfusion/volume contraction), there is increased proximal reabsorption of all solutes including urea, but creatinine excretion remains relatively preserved. This causes a disproportionate rise in BUN, pushing the ratio above 20:1. In true renal disease, both BUN and creatinine rise together, maintaining the normal ratio.

— Henry's Clinical Diagnosis and Management by Laboratory Methods; Costanzo Physiology, 7e

Q3. Why does creatinine clearance slightly overestimate true GFR?

A) Creatinine is partially reabsorbed in the proximal tubule B) Creatinine is secreted into the tubular lumen, adding to the filtered amount C) Creatinine freely diffuses across the glomerular membrane D) Creatinine is bound to plasma proteins

Answer & Explanation

✅ B) Creatinine is secreted into the tubular lumen, adding to the filtered amount

Creatinine is freely filtered but also undergoes a small degree of tubular secretion in the proximal tubule. This adds extra creatinine to the urine beyond what was filtered, making the calculated clearance (Ccr = Ucr × V / Pcr) slightly higher than the true GFR. With declining kidney function this overestimation increases due to progressive tubular secretion.

— Costanzo Physiology, 7e; Morgan & Mikhail's Clinical Anesthesiology, 7e

Q4. A patient on a fluid-restricted diet has BUN 60 mg/dL and creatinine 3.5 mg/dL. Their urine-to-plasma osmolality ratio (Uosm/Posm) is 0.9. Where is the likely lesion?

A) Glomerulus B) Renal tubules C) Renal artery (prerenal) D) Ureter (postrenal obstruction)

Answer & Explanation

✅ B) Renal tubules

Normal individuals on fluid restriction should have Uosm/Posm > 1.2, reflecting tubular concentration ability. A ratio < 1.2 indicates failure to concentrate urine, pointing to a tubular lesion (e.g., acute tubular necrosis, pyelonephritis, ischemia). If the ratio were normal (>1.2), the lesion would be glomerular, confirmed by proteinuria/haematuria on urinalysis.

— Henry's Clinical Diagnosis and Management by Laboratory Methods

Q5. Which of the following statements about Blood Urea Nitrogen (BUN) is CORRECT?

A) BUN is entirely dependent on GFR and unaffected by diet B) BUN is a more specific marker of GFR than serum creatinine C) BUN can be elevated by high protein intake, GI bleeding, and dehydration D) BUN is unaffected by liver disease

Answer & Explanation

✅ C) BUN can be elevated by high protein intake, GI bleeding, and dehydration

Unlike creatinine, BUN is influenced by multiple extrarenal factors: dietary protein intake (high protein → more urea production), GI bleeding (blood protein absorbed and converted to urea), hydration status, and liver function (liver synthesizes urea from ammonia — liver disease lowers BUN). Malnutrition or liver disease may decrease BUN even with impaired renal function, making it a less specific marker than creatinine.

— Smith and Tanagho's General Urology, 19e; Brenner & Rector's The Kidney, 2e

Q6. The GFR formula using creatinine clearance is:

A) GFR = Pcr × V / Ucr B) GFR = Ucr × V / Pcr C) GFR = Ucr / (Pcr × V) D) GFR = Pcr / (Ucr × V)

Answer & Explanation

✅ B) GFR = Ucr × V / Pcr

The creatinine clearance equation derives from the principle that the amount of creatinine excreted in urine (Ucr × V, where V = urine volume per unit time) equals the amount filtered (GFR × Pcr). Rearranging: GFR = Ccr = Ucr × V / Pcr. This represents the volume of plasma completely cleared of creatinine per unit time.

— Henry's Clinical Diagnosis and Management by Laboratory Methods

Q7. Which endogenous substance is considered the closest practical alternative to inulin for estimating GFR?

A) Cystatin C B) Serum creatinine C) Beta-2 microglobulin D) SDMA (symmetric dimethylarginine)

Answer & Explanation

✅ B) Serum creatinine

Among endogenous substances, creatinine is the closest practical alternative to inulin — it is freely filtered and net secretion and reabsorption roughly cancel each other out over a wide GFR range. While cystatin C is less influenced by muscle mass, age, and sex (and performs better at higher GFR levels), creatinine remains the most widely used clinical tool because it is endogenous and does not require exogenous infusion.

— Costanzo Physiology, 7e; Tietz Textbook of Laboratory Medicine, 7e

Q8. A urine specific gravity of 1.030 and osmolality of ~1050 mOsm/kg indicates:

A) Isosthenuria (tubular failure) B) Excellent tubular concentrating function C) Nephrotic syndrome D) Prerenal azotemia

Answer & Explanation

✅ B) Excellent tubular concentrating function

A urinary specific gravity of 1.030 (osmolality ≈1050 mOsm/kg) indicates maximum concentrating ability by the renal tubules — a sign of intact tubular function. Conversely, a urinary osmolality fixed near that of plasma (specific gravity ~1.010, osmolality ~285–300 mOsm/kg) — called isosthenuria — indicates loss of tubular concentrating ability and suggests significant tubular damage.

— Miller's Anesthesia, 10e; Morgan & Mikhail's Clinical Anesthesiology, 7e

Q9. In a patient with nephrotic syndrome, urine protein electrophoresis would most likely show:

A) Multiple bands — albumin, α, β, and γ globulins B) Predominantly albumin only C) Predominantly γ-globulins (Bence Jones proteins) D) No protein (protein excretion is normal)

Answer & Explanation

✅ B) Predominantly albumin only

Nephrotic syndrome (also called "nephrotic pattern" or minimal change disease) involves selective damage to the glomerular filtration barrier, allowing only albumin to pass through due to its size. Urine protein electrophoresis shows predominantly albumin. By contrast, the nephritic pattern (morphologically damaged glomerulus) shows multiple protein bands (albumin + α, β, γ globulins) because the damaged glomerulus is non-selective and allows larger proteins through.

— Henry's Clinical Diagnosis and Management by Laboratory Methods

Q10. Cystatin C has an advantage over serum creatinine for GFR estimation because:

A) It is secreted by the proximal tubule and thus more accurately reflects filtration B) It is less influenced by age, sex, and muscle mass C) It is produced exclusively in the kidney D) It can be measured only in urine, reflecting tubular function

Answer & Explanation

✅ B) It is less influenced by age, sex, and muscle mass

Cystatin C is a low-molecular-weight protein produced by all nucleated cells at a constant rate. Unlike creatinine, its serum concentration is not significantly affected by muscle mass, age, or sex — variables that confound creatinine-based GFR estimates. Cystatin C also better estimates GFR at higher GFR levels (where serum creatinine is less sensitive) and performs superiorly in frail or elderly patients, pediatric populations, and amputees.

— National Kidney Foundation Primer on Kidney Diseases, 8e; Tietz Textbook of Laboratory Medicine, 7e

Quick Reference Summary

MCQ	Topic	Key Fact
1	Gold standard GFR	Inulin clearance
2	BUN/Cr ratio	>20:1 → prerenal; 10–20:1 → renal/postrenal
3	Cr clearance overestimates	Tubular secretion of creatinine
4	Uosm/Posm ratio	<1.2 → tubular lesion
5	BUN confounders	Diet, GI bleeding, hydration, liver disease
6	Ccr formula	Ucr × V / Pcr
7	Practical GFR marker	Serum creatinine
8	Specific gravity 1.030	Excellent tubular function
9	Nephrotic vs nephritic	Nephrotic = albumin only
10	Cystatin C advantage	Independent of muscle mass/age/sex

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