A 22-year-old woman is brought to casualty during a panic attack after a family quarrel. She is breathing fast and has tingling around the mouth and carpopedal spasm. ABG: pH 7.55, PaCO2 25 mmHg, HCO3- 21 mmol/L. Total serum calcium is 9.4 mg/dL. Why does she have tetany?

More calcium binds to albumin, so ionised calcium falls

Hyperventilation suppresses parathyroid hormone release

Alkalosis increases renal excretion of calcium

Low PaCO2 moves calcium into bone

A 55-year-old woman has taken pantoprazole 40 mg daily for three years for reflux. During workup of occasional facial flushing, her serum chromogranin A is four times the upper limit. Her 24-hour urine 5-HIAA and contrast CT of the abdomen are normal. What is the next best step?

Repeat chromogranin A after stopping the PPI

Order a Ga-68 DOTATATE PET scan

Refer for diagnostic laparoscopy

Start octreotide for a suspected carcinoid

Clinical biochemistry: acid-base, electrolytes and organ function tests

15 NEET-PG practice questions on Clinical biochemistry: acid-base, electrolytes and organ function tests, part of Biochemistry. The first 15 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 woman is brought to casualty during a panic attack after a family quarrel. She is breathing fast and has tingling around the mouth and carpopedal spasm. ABG: pH 7.55, PaCO2 25 mmHg, HCO3- 21 mmol/L. Total serum calcium is 9.4 mg/dL. Why does she have tetany?
1. AHyperventilation suppresses parathyroid hormone release
2. BAlkalosis increases renal excretion of calcium
3. CLow PaCO2 moves calcium into bone
4. DMore calcium binds to albumin, so ionised calcium falls
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Answer: D · More calcium binds to albumin, so ionised calcium falls
The ABG shows acute respiratory alkalosis: a low PaCO2 with a high pH, and bicarbonate only slightly lowered by acute buffering (about 2 mmol/L per 10 mmHg fall). When blood becomes alkaline, albumin releases hydrogen ions and binds more calcium, so the ionised fraction falls although total calcium is normal. Low ionised calcium increases nerve excitability, causing perioral tingling and carpopedal spasm. Calm reassurance and slowing her breathing correct it; IV calcium is not needed.
- A: PTH is not suppressed by hyperventilation; low ionised calcium would stimulate it.
- B: Renal calcium loss is not the cause of acute tetany within minutes.
- C: Calcium does not shift into bone within minutes; protein binding explains the change.
Question 2 · Easy
A 64-year-old man has painless, progressive jaundice with weight loss and a palpable, non-tender gallbladder. CT shows a mass in the head of the pancreas. Which serum tumour marker is used with this tumour, mainly to follow treatment?
1. AAlpha-fetoprotein
2. BCA 19-9
3. CCA-125
4. DCalcitonin
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Answer: B · CA 19-9
Painless obstructive jaundice with a palpable gallbladder (Courvoisier's law) and a mass in the head of the pancreas suggests pancreatic adenocarcinoma. CA 19-9, a sialylated Lewis-a antigen, is the marker used, mainly to follow response and detect recurrence rather than to diagnose or screen. It can rise in benign biliary obstruction and cholangitis, and people who are Lewis antigen-negative cannot make it. Diagnosis rests on imaging and tissue.
- A: AFP is used for hepatocellular carcinoma and yolk sac tumours.
- C: CA-125 is mainly used for epithelial ovarian cancer.
- D: Calcitonin is the marker for medullary thyroid carcinoma.
Question 3 · Medium
A 55-year-old woman has taken pantoprazole 40 mg daily for three years for reflux. During workup of occasional facial flushing, her serum chromogranin A is four times the upper limit. Her 24-hour urine 5-HIAA and contrast CT of the abdomen are normal. What is the next best step?
1. AOrder a Ga-68 DOTATATE PET scan
2. BRefer for diagnostic laparoscopy
3. CRepeat chromogranin A after stopping the PPI
4. DStart octreotide for a suspected carcinoid
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Answer: C · Repeat chromogranin A after stopping the PPI
Chromogranin A is a general marker of neuroendocrine tumours, but proton pump inhibitors raise it: acid suppression increases gastrin, which stimulates enterochromaffin-like cells of the stomach to make chromogranin A. With normal 5-HIAA and imaging, the PPI is the likely cause, so the test should be repeated after the PPI has been stopped for an adequate period. Renal failure and atrophic gastritis also raise it falsely. Acting on a false-positive marker exposes the patient to needless tests and treatment.
- A: Functional imaging comes only if the marker stays high off the PPI.
- B: Laparoscopy is invasive and unjustified without evidence of a tumour.
- D: Octreotide treats a proven functioning NET, not an unexplained marker.
Question 4 · Medium
A 25-year-old woman with normal BP has had recurrent muscle weakness. Serum K+ is 2.8 mmol/L and HCO3- 32 mmol/L. She denies taking any medicine. She has dental enamel erosion and calluses on the backs of her fingers. Urine chloride is 8 mmol/L. What is the most likely cause?
1. AGitelman syndrome
2. BSelf-induced vomiting
3. CCovert diuretic use
4. DBartter syndrome
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Answer: B · Self-induced vomiting
Hypokalaemic metabolic alkalosis with normal BP has two main groups: loss of gastric acid, and renal salt wasting (diuretics, Bartter, Gitelman). Urine chloride separates them: it is low (usually below about 20 mmol/L) after vomiting, because the volume-depleted kidney conserves chloride, but high in diuretic use and the tubulopathies. Dental erosion and knuckle calluses (Russell's sign) suggest bulimia nervosa. She needs sensitive, non-judgemental counselling and referral to psychiatry.
- A: Gitelman syndrome wastes chloride, so urine chloride is high.
- C: Diuretics raise urine chloride while they are acting.
- D: Bartter syndrome also gives high urine chloride.
Question 5 · Medium
A 40-year-old man from a village in Gujarat is brought 18 hours after drinking illicit country liquor with friends, several of whom are also ill. He has blurred vision and vomiting. pH 7.08, HCO3- 6 mmol/L, Na+ 140 mmol/L, Cl- 100 mmol/L, glucose 90 mg/dL, urea 28 mg/dL and measured serum osmolality 330 mOsm/kg. Which metabolite is chiefly responsible for his eye damage?
1. AAcetaldehyde
2. BGlycolic acid
3. COxalic acid
4. DFormic acid
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Answer: D · Formic acid
An outbreak after illicit liquor, visual symptoms and a high anion gap acidosis (140 - 106 = 34 mmol/L) with a raised osmolal gap (calculated 2 x 140 + 90/18 + 28/6, about 290, so the gap is about 40) point to methanol poisoning. Alcohol dehydrogenase converts methanol to formaldehyde and then to formic acid, which inhibits cytochrome oxidase in the optic nerve and retina and causes the acidosis. Treatment is fomepizole or ethanol to block alcohol dehydrogenase, bicarbonate, folinic acid to speed formate clearance, and haemodialysis.
- A: Acetaldehyde is a metabolite of ethanol, not methanol.
- B: Glycolic acid is the main acid of ethylene glycol poisoning.
- C: Oxalic acid comes from ethylene glycol and damages the kidney, not the eye.
Question 6 · Medium
Routine blood from a 45-year-old woman at an executive health check shows K+ 8.1 mmol/L, total calcium 3.2 mg/dL and ALP 18 U/L (reference 40-130 U/L). She feels well, has no tetany, and her ECG is normal. The sample was drawn after the CBC tube in the same sitting. What is the most likely cause?
1. AContamination with K2-EDTA from a CBC tube
2. BOvernight storage of whole blood in a fridge
3. CPseudohypoparathyroidism with renal failure
4. DIn vitro haemolysis of the sample
Answer and explanation in LearnFree account
Question 7 · Easy
A 20-year-old student is found unconscious in a hostel room with pinpoint pupils and a respiratory rate of 6/min. ABG on room air: pH 7.21, PaCO2 68 mmHg, HCO3- 26 mmol/L. What is the acid-base disorder?
1. AChronic respiratory acidosis
2. BMixed respiratory and metabolic acidosis
3. CAcute respiratory acidosis
4. DMetabolic acidosis with compensation
Answer and explanation in LearnFree account
Question 8 · Medium
A 70-year-old woman, bedridden for a year after a stroke, has marked muscle wasting and weighs 38 kg. Her serum creatinine is 0.7 mg/dL. Before starting a drug that is cleared by the kidneys, the physician suspects her true GFR is lower than the creatinine suggests. Which marker is least affected by her low muscle mass?
1. ASerum uric acid
2. BBlood urea
3. CUrine specific gravity
4. DSerum cystatin C
Answer and explanation in LearnFree account
Question 9 · Hard
A 45-year-old man has been drinking heavily and eating little for a week, with persistent vomiting for the last 4 days. ABG: pH 7.36, PaCO2 40 mmHg, HCO3- 22 mmol/L. Na+ 140 mmol/L, K+ 3.0 mmol/L, Cl- 90 mmol/L, glucose 110 mg/dL, lactate 2 mmol/L, and serum beta-hydroxybutyrate is raised (anion gap reference 8-12 mmol/L). How is his acid-base status best described?
1. ANo significant acid-base disorder
2. BRespiratory acidosis with metabolic alkalosis
3. CHigh anion gap acidosis alone, compensated
4. DHigh anion gap acidosis with metabolic alkalosis
Answer and explanation in LearnFree account
Question 10 · Easy
A 19-year-old student with hepatitis A for 10 days becomes drowsy and confused. Which single test best indicates the severity of his liver failure?
1. ASerum alkaline phosphatase
2. BSerum ALT
3. CProthrombin time (INR)
4. DSerum albumin
Answer and explanation in LearnFree account
Question 11 · Medium
A 42-year-old woman took amoxicillin-clavulanate for 10 days for sinusitis. Two weeks later she has jaundice and itching. ALT is 96 U/L (upper limit 40 U/L), ALP 520 U/L (upper limit 130 U/L) and bilirubin 5.4 mg/dL, mostly direct. Ultrasound shows no bile duct dilatation. Using the R ratio, how is the liver injury classified?
1. AHaemolytic (R not applicable)
2. BHepatocellular (R above 5)
3. CMixed (R between 2 and 5)
4. DCholestatic (R below 2)
Answer and explanation in LearnFree account
Question 12 · Hard
A 52-year-old woman had a Whipple operation for pancreatic adenocarcinoma 8 months ago. Her CA 19-9 before surgery was undetectable (below 2 U/mL) despite a large tumour. During follow-up, the oncologist considers four statements: 1. She may be Lewis antigen-negative, so CA 19-9 cannot track her disease. 2. Biliary obstruction or cholangitis can raise CA 19-9 without recurrence. 3. A normal CA 19-9 would rule out recurrence in her. 4. Her follow-up should rely on imaging. Which statements are correct?
1. A2 and 3 only
2. B1, 2 and 3 only
3. C1 and 4 only
4. D1, 2 and 4 only
Answer and explanation in LearnFree account
Question 13 · Hard
A 28-year-old fashion model with normal BP has weakness and fatigue. ABG: pH 7.32, PaCO2 30 mmHg, HCO3- 15 mmol/L. Serum Na+ 138 mmol/L, K+ 2.9 mmol/L, Cl- 114 mmol/L. Spot urine: Na+ 25 mmol/L, K+ 15 mmol/L, Cl- 75 mmol/L. She denies diarrhoea, and her blood glucose is normal. What is the most likely cause?
1. ASurreptitious laxative abuse
2. BDistal (type 1) renal tubular acidosis
3. CType 4 renal tubular acidosis
4. DDiabetic ketoacidosis
Answer and explanation in LearnFree account
Question 14 · Medium
A 64-year-old man with back pain and Hb 8.4 g/dL has serum total protein 11.6 g/dL (albumin 2.9 g/dL). The laboratory reports Na+ 127 mmol/L, measured by indirect ion-selective electrode on the main analyser. Measured serum osmolality is 289 mOsm/kg, glucose 96 mg/dL and urea 30 mg/dL. He has no neurological symptoms. What is the next best step?
1. AMeasure sodium by direct ion-selective electrode
2. BCorrect the sodium for the glucose level
3. CRestrict fluid to 800 mL a day
4. DInfuse 3% hypertonic saline
Answer and explanation in LearnFree account
Question 15 · Medium
A 35-year-old labourer from a cholera-affected area of Kolkata is admitted with profuse watery diarrhoea for a day. ABG: pH 7.27, PaCO2 29 mmHg, HCO3- 13 mmol/L. Serum Na+ 138 mmol/L, K+ 3.1 mmol/L, Cl- 114 mmol/L (anion gap reference 8-12 mmol/L). How is his acid-base status best described?
1. ARespiratory alkalosis with renal compensation
2. BNormal anion gap metabolic acidosis
3. CHigh anion gap metabolic acidosis
4. DMetabolic acidosis with respiratory acidosis
Answer and explanation in LearnFree account

Clinical biochemistry: acid-base, electrolytes and organ function tests

A 22-year-old woman is brought to casualty during a panic attack after a family quarrel. She is breathing fast and has tingling around the mouth and carpopedal spasm. ABG: pH 7.55, PaCO2 25 mmHg, HCO3- 21 mmol/L. Total serum calcium is 9.4 mg/dL. Why does she have tetany?

A 64-year-old man has painless, progressive jaundice with weight loss and a palpable, non-tender gallbladder. CT shows a mass in the head of the pancreas. Which serum tumour marker is used with this tumour, mainly to follow treatment?

A 55-year-old woman has taken pantoprazole 40 mg daily for three years for reflux. During workup of occasional facial flushing, her serum chromogranin A is four times the upper limit. Her 24-hour urine 5-HIAA and contrast CT of the abdomen are normal. What is the next best step?

A 25-year-old woman with normal BP has had recurrent muscle weakness. Serum K+ is 2.8 mmol/L and HCO3- 32 mmol/L. She denies taking any medicine. She has dental enamel erosion and calluses on the backs of her fingers. Urine chloride is 8 mmol/L. What is the most likely cause?

Routine blood from a 45-year-old woman at an executive health check shows K+ 8.1 mmol/L, total calcium 3.2 mg/dL and ALP 18 U/L (reference 40-130 U/L). She feels well, has no tetany, and her ECG is normal. The sample was drawn after the CBC tube in the same sitting. What is the most likely cause?

A 20-year-old student is found unconscious in a hostel room with pinpoint pupils and a respiratory rate of 6/min. ABG on room air: pH 7.21, PaCO2 68 mmHg, HCO3- 26 mmol/L. What is the acid-base disorder?

A 19-year-old student with hepatitis A for 10 days becomes drowsy and confused. Which single test best indicates the severity of his liver failure?

A 35-year-old labourer from a cholera-affected area of Kolkata is admitted with profuse watery diarrhoea for a day. ABG: pH 7.27, PaCO2 29 mmHg, HCO3- 13 mmol/L. Serum Na+ 138 mmol/L, K+ 3.1 mmol/L, Cl- 114 mmol/L (anion gap reference 8-12 mmol/L). How is his acid-base status best described?