A 22-year-old with type 1 diabetes is brought to casualty after missing insulin doses. ABG: pH 7.15, PaCO2 23 mmHg, HCO3- 10 mmol/L. Serum Na+ 135 mmol/L, K+ 5.2 mmol/L, Cl- 98 mmol/L. How is the acid-base disorder best described?

High anion gap metabolic acidosis, appropriately compensated

High anion gap metabolic acidosis with respiratory alkalosis

High anion gap metabolic acidosis with respiratory acidosis

Normal anion gap metabolic acidosis, appropriately compensated

A 5-week-old boy has had projectile non-bilious vomiting after every feed for a week, and an olive-shaped mass is felt in the epigastrium. Results: pH 7.50, PaCO2 48 mmHg, HCO3- 36 mmol/L, Na+ 133 mmol/L, K+ 2.9 mmol/L and Cl- 84 mmol/L; his urine pH is 5.5. What sustains the acidic urine despite his alkalaemia?

Volume and chloride depletion driving distal H+ secretion

Hypoventilation raising PaCO2 and so tubular H+ secretion

Continuing gastric loss of bicarbonate-rich secretion

Distal renal tubular acidosis caused by hypokalaemia

A 35-year-old man with bipolar disorder has been on lithium for 6 years. He now passes about 5 L of urine a day and his serum Na+ is 146 mmol/L. After supervised water deprivation his urine osmolality is 180 mOsm/kg, and 2 hours after desmopressin it is 195 mOsm/kg. What is the mechanism of his polyuria?

Collecting duct unresponsiveness to ADH

Excess water intake suppressing ADH

Osmotic diuresis from glycosuria

Failure of ADH release from the pituitary

A 68-year-old hypertensive diabetic man is started on ramipril. A week later his serum creatinine has risen from 1.2 to 2.4 mg/dL, and Doppler later shows severe stenosis of both renal arteries. What is the mechanism of his fall in GFR?

Loss of angiotensin II tone at the efferent arteriole

Constriction of the afferent arteriole by bradykinin

Raised hydrostatic pressure in Bowman's capsule

Allergic interstitial nephritis caused by the ramipril

Renal physiology and acid-base

23 NEET-PG practice questions on Renal physiology and acid-base, part of Physiology. The first 23 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A 22-year-old with type 1 diabetes is brought to casualty after missing insulin doses. ABG: pH 7.15, PaCO2 23 mmHg, HCO3- 10 mmol/L. Serum Na+ 135 mmol/L, K+ 5.2 mmol/L, Cl- 98 mmol/L. How is the acid-base disorder best described?
1. AHigh anion gap metabolic acidosis with respiratory alkalosis
2. BHigh anion gap metabolic acidosis with respiratory acidosis
3. CHigh anion gap metabolic acidosis, appropriately compensated
4. DNormal anion gap metabolic acidosis, appropriately compensated
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Answer: C · High anion gap metabolic acidosis, appropriately compensated
Low pH with low bicarbonate is metabolic acidosis. The anion gap is Na+ - (Cl- + HCO3-) = 135 - (98 + 10) = 27 mmol/L, which is high (normal about 8-12), consistent with ketoacids in diabetic ketoacidosis. Winters' formula predicts PaCO2 = 1.5 x HCO3- + 8 = 23 +/- 2 mmHg, which matches the measured 23, so respiratory compensation is appropriate and there is no second respiratory disorder. A PaCO2 above or below that range would point to an added respiratory acidosis or alkalosis.
- A: Respiratory alkalosis would need a PaCO2 below about 21 mmHg here.
- B: Respiratory acidosis would need a PaCO2 above about 25 mmHg here.
- D: The anion gap here is 27, so it is not a normal anion gap acidosis.
Question 2 · Hard
A 5-week-old boy has had projectile non-bilious vomiting after every feed for a week, and an olive-shaped mass is felt in the epigastrium. Results: pH 7.50, PaCO2 48 mmHg, HCO3- 36 mmol/L, Na+ 133 mmol/L, K+ 2.9 mmol/L and Cl- 84 mmol/L; his urine pH is 5.5. What sustains the acidic urine despite his alkalaemia?
1. AHypoventilation raising PaCO2 and so tubular H+ secretion
2. BContinuing gastric loss of bicarbonate-rich secretion
3. CDistal renal tubular acidosis caused by hypokalaemia
4. DVolume and chloride depletion driving distal H+ secretion
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Answer: D · Volume and chloride depletion driving distal H+ secretion
Vomiting gastric HCl in hypertrophic pyloric stenosis causes a hypochloraemic, hypokalaemic metabolic alkalosis. Volume depletion stimulates the renin-angiotensin-aldosterone system, and with little chloride available the kidney reabsorbs Na+ in exchange for H+ and K+ in the distal nephron, and cannot excrete the excess bicarbonate. The result is 'paradoxical aciduria', which worsens the alkalosis and hypokalaemia. Rehydration with 0.9% saline and potassium corrects it before pyloromyotomy, since anaesthesia with an uncorrected alkalosis risks post-operative apnoea.
- A: His PaCO2 of 48 is the expected respiratory compensation; it is not what keeps the urine acid.
- B: Gastric juice is rich in HCl, not bicarbonate; its loss causes the alkalosis.
- C: Hypokalaemia does not cause distal RTA; distal RTA gives acidosis with alkaline urine.
Question 3 · Medium
A 35-year-old man with bipolar disorder has been on lithium for 6 years. He now passes about 5 L of urine a day and his serum Na+ is 146 mmol/L. After supervised water deprivation his urine osmolality is 180 mOsm/kg, and 2 hours after desmopressin it is 195 mOsm/kg. What is the mechanism of his polyuria?
1. AExcess water intake suppressing ADH
2. BOsmotic diuresis from glycosuria
3. CCollecting duct unresponsiveness to ADH
4. DFailure of ADH release from the pituitary
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Answer: C · Collecting duct unresponsiveness to ADH
Urine that stays dilute after water deprivation and barely rises after desmopressin means the kidney cannot respond to ADH: nephrogenic diabetes insipidus. Lithium enters principal cells through epithelial sodium channels (ENaC) and impairs V2 signalling and aquaporin-2 expression. Amiloride, which blocks ENaC and so limits lithium entry, can reduce the polyuria if lithium must continue.
- A: In primary polydipsia, water deprivation concentrates the urine and serum sodium is low-normal, not high.
- B: Osmotic diuresis gives urine osmolality near or above plasma, not 180 mOsm/kg.
- D: In central DI, urine osmolality rises sharply (well over 50%) after desmopressin.
Question 4 · Easy
Clearance of which substance, infused at a low plasma level, is used to estimate effective renal plasma flow?
1. ACreatinine
2. BInulin
3. CPara-aminohippuric acid
4. DIohexol
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Answer: C · Para-aminohippuric acid
At low plasma levels, para-aminohippuric acid (PAH) is filtered and then secreted by the proximal tubule so efficiently that about 90% is cleared from the plasma in one pass. Its clearance therefore approximates renal plasma flow; because extraction is not complete, it is called effective renal plasma flow. Renal blood flow is then derived by dividing by (1 - haematocrit).
- A: Creatinine clearance approximates GFR, slightly overestimating it.
- B: Inulin is filtered but neither reabsorbed nor secreted, so its clearance measures GFR.
- D: Iohexol plasma clearance is used to measure GFR, not plasma flow.
Question 5 · Medium
A 68-year-old hypertensive diabetic man is started on ramipril. A week later his serum creatinine has risen from 1.2 to 2.4 mg/dL, and Doppler later shows severe stenosis of both renal arteries. What is the mechanism of his fall in GFR?
1. AConstriction of the afferent arteriole by bradykinin
2. BLoss of angiotensin II tone at the efferent arteriole
3. CRaised hydrostatic pressure in Bowman's capsule
4. DAllergic interstitial nephritis caused by the ramipril
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Answer: B · Loss of angiotensin II tone at the efferent arteriole
When perfusion pressure falls behind a stenosis, angiotensin II preferentially constricts the efferent arteriole, which keeps glomerular capillary hydrostatic pressure, and hence GFR, from falling. An ACE inhibitor removes this efferent tone, so glomerular pressure and GFR drop sharply. This is why ACE inhibitors and ARBs are avoided in bilateral renal artery stenosis (or stenosis to a single kidney), and creatinine is checked within 1-2 weeks of starting them.
- A: Bradykinin, which rises with ACE inhibition, is a vasodilator and does not constrict the afferent arteriole.
- C: Bowman's capsule pressure rises with urinary obstruction, not with ACE inhibition.
- D: Drug-induced interstitial nephritis would not depend on the renal artery stenosis and usually brings rash, fever or pyuria.
Question 6 · Medium
A 3-year-old girl recovering from severe acute malnutrition at a nutrition rehabilitation centre has been on a very low-protein diet. Her serum Na+, K+ and calcium are normal, but after an overnight fluid restriction her urine osmolality reaches only 450 mOsm/kg. Which deficit best explains her reduced ability to concentrate urine?
1. AFailure of ADH release from the pituitary
2. BAbsent aquaporin-2 channels in the collecting ducts
3. CLoss of NKCC2 activity in the thick limb
4. DLess urea in the inner medullary interstitium
Answer and explanation in LearnFree account
Question 7 · Easy
At an NCD screening camp, a 48-year-old shopkeeper with untreated diabetes has glucose 3+ on urine dipstick and a plasma glucose of 260 mg/dL. Above roughly what plasma glucose level does glucose usually start to appear in the urine of an adult with normal kidneys?
1. AAbout 375 mg/dL
2. BAbout 126 mg/dL
3. CAbout 110 mg/dL
4. DAbout 180 mg/dL
Answer and explanation in LearnFree account
Question 8 · Easy
In tubuloglomerular feedback, which cells sense the sodium chloride concentration of the tubular fluid?
1. AMacula densa cells
2. BJuxtaglomerular granular cells
3. CCollecting duct principal cells
4. DExtraglomerular mesangial cells
Answer and explanation in LearnFree account
Question 9 · Easy
A 25-year-old motorcyclist is brought to casualty after a road accident with a fractured femur and a pulse of 124/min. His urine output is low and urine osmolality is 900 mOsm/kg. Which water channel has been inserted into the luminal membrane of his collecting duct principal cells?
1. AAquaporin-1
2. BAquaporin-3
3. CAquaporin-2
4. DAquaporin-4
Answer and explanation in LearnFree account
Question 10 · Hard
In a renal physiology laboratory, a 26-year-old volunteer with a haematocrit of 45% receives constant infusions of inulin and PAH. Urine flow is 1.0 mL/min. Urine inulin is 30 mg/mL and plasma inulin 0.25 mg/mL; urine PAH is 15 mg/mL and plasma PAH 0.025 mg/mL. What is his filtration fraction?
1. A0.11
2. B0.09
3. C5.0
4. D0.20
Answer and explanation in LearnFree account
Question 11 · Medium
A 64-year-old bidi smoker with COPD is reviewed in the chest OPD, where he is stable. His arterial blood gas on room air shows pH 7.35, PaCO2 60 mmHg and HCO3- 32 mmol/L. What is the best interpretation?
1. AAcute respiratory acidosis
2. BRespiratory acidosis with metabolic acidosis
3. CMetabolic alkalosis with appropriate respiratory compensation
4. DChronic respiratory acidosis with renal compensation
Answer and explanation in LearnFree account
Question 12 · Hard
A 45-year-old daily-wage labourer from Thrissur, a heavy toddy drinker, is brought by his brother after four days of persistent retching with no food intake. He is tachycardic with dry mucosa; capillary glucose is 118 mg/dL and urine ketones are strongly positive. Arterial pH 7.37, PaCO2 40 mmHg, HCO3- 22 mmol/L; Na+ 140, K+ 3.2 and Cl- 88 mmol/L. Assume a normal anion gap of 12 and a normal bicarbonate of 24 mmol/L. How should his acid-base status be described?
1. AHigh anion gap acidosis with respiratory acidosis
2. BHigh anion gap acidosis with metabolic alkalosis
3. CNormal anion gap acidosis with metabolic alkalosis
4. DNo acid-base disorder, as the pH is normal
Answer and explanation in LearnFree account
Question 13 · Hard
A 58-year-old man with small cell carcinoma of the lung has serum Na+ 124 mmol/L and plasma osmolality 255 mOsm/kg. His urine osmolality is 510 mOsm/kg and urine flow is 1.5 mL/min. What is his free water clearance?
1. A+1.5 mL/min
2. B-1.5 mL/min
3. C+4.5 mL/min
4. D-3.0 mL/min
Answer and explanation in LearnFree account
Question 14 · Medium
A 38-year-old man has a 9 mm stone impacted at the left vesico-ureteric junction for 10 days, with left hydroureteronephrosis. A renal scan shows reduced filtration in the left kidney. Which Starling force is directly raised by the obstruction and lowers his GFR?
1. AHydrostatic pressure in Bowman's space
2. BOncotic pressure in glomerular capillaries
3. CHydrostatic pressure in peritubular capillaries
4. DOncotic pressure in Bowman's space
Answer and explanation in LearnFree account
Question 15 · Medium
A 32-year-old woman with dry eyes and dry mouth has had three renal stones. Results: Na+ 139 mmol/L, K+ 2.9 mmol/L, Cl- 115 mmol/L, HCO3- 15 mmol/L, arterial pH 7.30, and urine pH 6.8. Consider four statements about her disorder: 1. The defect is in H+ secretion by alpha-intercalated cells. 2. Her anion gap is normal. 3. High urinary citrate explains her stones. 4. Hypokalaemia is expected in this type of renal tubular acidosis. Which statements are correct?
1. A1, 2 and 4 only
2. B2 and 4 only
3. C1 and 3 only
4. D1, 2 and 3 only
Answer and explanation in LearnFree account
Question 16 · Medium
A 30-year-old prospective kidney donor in Chennai has measured clearance studies. Consider four statements: 1. Inulin clearance equals GFR because inulin is freely filtered and neither reabsorbed nor secreted. 2. Creatinine clearance underestimates GFR because some creatinine is secreted. 3. A substance whose clearance exceeds inulin clearance undergoes net tubular secretion. 4. At a normal plasma glucose, glucose clearance is close to zero. Which statements are correct?
1. A1 and 3 only
2. B1, 3 and 4 only
3. C1 and 2 only
4. D3 and 4 only
Answer and explanation in LearnFree account
Question 17 · Medium
A 2-year-old boy from a village in Bihar has had watery diarrhoea for 3 days and has some dehydration. Results: Na+ 136 mmol/L, K+ 3.1 mmol/L, Cl- 114 mmol/L, HCO3- 12 mmol/L, pH 7.24 and PaCO2 26 mmHg (take a normal anion gap as 8-12 mmol/L). Which acid-base disorder does he have?
1. ANormal anion gap metabolic acidosis
2. BMetabolic acidosis with respiratory acidosis
3. CMetabolic acidosis with respiratory alkalosis
4. DHigh anion gap metabolic acidosis
Answer and explanation in LearnFree account
Question 18 · Medium
A 60-year-old woman with heart failure is on high-dose furosemide. During a febrile illness with poor fluid intake, she still passes large volumes of urine with an osmolality close to that of plasma, although her ADH level is high. Why can she not concentrate her urine?
1. AThe medullary interstitial gradient is lost
2. BThe collecting duct cannot insert aquaporin-2
3. CPosterior pituitary ADH release is suppressed
4. DThe thin descending limb loses aquaporin-1
Answer and explanation in LearnFree account
Question 19 · Easy
A 32-year-old trekker from Kerala is given acetazolamide before flying to Leh to prevent acute mountain sickness. The drug inhibits carbonic anhydrase mainly in the nephron segment that reclaims most of the filtered bicarbonate. Which segment is this?
1. AProximal convoluted tubule
2. BThick ascending limb of Henle
3. CDistal convoluted tubule
4. DCortical collecting duct
Answer and explanation in LearnFree account
Question 20 · Medium
A 34-year-old man with HIV infection is started on cotrimoxazole prophylaxis. Two weeks later his serum creatinine has risen from 0.8 to 1.1 mg/dL. Urinalysis is normal, blood pressure is normal, and an eGFR based on cystatin C is unchanged. What best explains the rise in creatinine?
1. AInhibition of tubular creatinine secretion
2. BA true fall in the glomerular filtration rate
3. CTubular obstruction by sulfonamide crystals
4. DIncreased release of creatinine from muscle
Answer and explanation in LearnFree account
Question 21 · Medium
A 55-year-old man with type 2 diabetes and heart failure is started on dapagliflozin. At review his urine glucose is 3+ although his plasma glucose is 140 mg/dL. How does the drug produce glycosuria?
1. AIt blocks SGLT2 in the early proximal tubule
2. BIt lowers the filtration rate of every single nephron
3. CIt blocks GLUT2 on the basolateral membrane
4. DIt blocks SGLT1 in the late proximal tubule
Answer and explanation in LearnFree account
Question 22 · Medium
A 26-year-old woman in the medicine OPD has BP 172/106 mmHg, serum K+ 3.1 mmol/L and a bruit to the right of the umbilicus. Plasma renin activity and serum aldosterone are both raised. What is the most likely cause of her hypertension?
1. ALiddle syndrome
2. B11-beta-hydroxylase deficiency
3. CAldosterone-producing adrenal adenoma
4. DFibromuscular dysplasia of a renal artery
Answer and explanation in LearnFree account
Question 23 · Hard
A 19-year-old college student is brought to casualty 6 hours after swallowing a large number of aspirin tablets. She has tinnitus and is breathing deeply at 30/min. Results: pH 7.47, PaCO2 20 mmHg, HCO3- 14 mmol/L, Na+ 140 mmol/L and Cl- 104 mmol/L (take a normal anion gap as 8-12 mmol/L). Which acid-base disturbance does she have?
1. ANormal anion gap acidosis with respiratory alkalosis
2. BPrimary respiratory alkalosis, renally compensated
3. CHigh anion gap acidosis with respiratory alkalosis
4. DHigh anion gap acidosis, fully compensated
Answer and explanation in LearnFree account

Renal physiology and acid-base

A 22-year-old with type 1 diabetes is brought to casualty after missing insulin doses. ABG: pH 7.15, PaCO2 23 mmHg, HCO3- 10 mmol/L. Serum Na+ 135 mmol/L, K+ 5.2 mmol/L, Cl- 98 mmol/L. How is the acid-base disorder best described?

Clearance of which substance, infused at a low plasma level, is used to estimate effective renal plasma flow?

A 68-year-old hypertensive diabetic man is started on ramipril. A week later his serum creatinine has risen from 1.2 to 2.4 mg/dL, and Doppler later shows severe stenosis of both renal arteries. What is the mechanism of his fall in GFR?

At an NCD screening camp, a 48-year-old shopkeeper with untreated diabetes has glucose 3+ on urine dipstick and a plasma glucose of 260 mg/dL. Above roughly what plasma glucose level does glucose usually start to appear in the urine of an adult with normal kidneys?

In tubuloglomerular feedback, which cells sense the sodium chloride concentration of the tubular fluid?

A 25-year-old motorcyclist is brought to casualty after a road accident with a fractured femur and a pulse of 124/min. His urine output is low and urine osmolality is 900 mOsm/kg. Which water channel has been inserted into the luminal membrane of his collecting duct principal cells?

In a renal physiology laboratory, a 26-year-old volunteer with a haematocrit of 45% receives constant infusions of inulin and PAH. Urine flow is 1.0 mL/min. Urine inulin is 30 mg/mL and plasma inulin 0.25 mg/mL; urine PAH is 15 mg/mL and plasma PAH 0.025 mg/mL. What is his filtration fraction?

A 64-year-old bidi smoker with COPD is reviewed in the chest OPD, where he is stable. His arterial blood gas on room air shows pH 7.35, PaCO2 60 mmHg and HCO3- 32 mmol/L. What is the best interpretation?

A 58-year-old man with small cell carcinoma of the lung has serum Na+ 124 mmol/L and plasma osmolality 255 mOsm/kg. His urine osmolality is 510 mOsm/kg and urine flow is 1.5 mL/min. What is his free water clearance?

A 38-year-old man has a 9 mm stone impacted at the left vesico-ureteric junction for 10 days, with left hydroureteronephrosis. A renal scan shows reduced filtration in the left kidney. Which Starling force is directly raised by the obstruction and lowers his GFR?

A 2-year-old boy from a village in Bihar has had watery diarrhoea for 3 days and has some dehydration. Results: Na+ 136 mmol/L, K+ 3.1 mmol/L, Cl- 114 mmol/L, HCO3- 12 mmol/L, pH 7.24 and PaCO2 26 mmHg (take a normal anion gap as 8-12 mmol/L). Which acid-base disorder does he have?

A 60-year-old woman with heart failure is on high-dose furosemide. During a febrile illness with poor fluid intake, she still passes large volumes of urine with an osmolality close to that of plasma, although her ADH level is high. Why can she not concentrate her urine?

A 32-year-old trekker from Kerala is given acetazolamide before flying to Leh to prevent acute mountain sickness. The drug inhibits carbonic anhydrase mainly in the nephron segment that reclaims most of the filtered bicarbonate. Which segment is this?

A 34-year-old man with HIV infection is started on cotrimoxazole prophylaxis. Two weeks later his serum creatinine has risen from 0.8 to 1.1 mg/dL. Urinalysis is normal, blood pressure is normal, and an eGFR based on cystatin C is unchanged. What best explains the rise in creatinine?

A 55-year-old man with type 2 diabetes and heart failure is started on dapagliflozin. At review his urine glucose is 3+ although his plasma glucose is 140 mg/dL. How does the drug produce glycosuria?

A 26-year-old woman in the medicine OPD has BP 172/106 mmHg, serum K+ 3.1 mmol/L and a bruit to the right of the umbilicus. Plasma renin activity and serum aldosterone are both raised. What is the most likely cause of her hypertension?