A 24-year-old man with hereditary spherocytosis is seen on day 9 of oral broad-spectrum antibiotics for a deep wound infection. Hemoglobin 10.6 g/dL [106 g/L], reticulocytes 8.5%, total bilirubin 3.9 mg/dL [67 µmol/L], direct bilirubin 0.3 mg/dL [5 µmol/L], ALP and ALT normal; these are close to his usual values. A freshly voided urine, protected from light and tested within 15 minutes, shows bilirubin negative and urobilinogen 0.2 EU/dL (the lowest pad block). Two months ago, with similar blood results, his urine urobilinogen was 4.0 EU/dL. What best explains the change?

Fewer colonic bacteria to convert bilirubin

Bile duct obstruction stopping bile flow

Less hemolysis reducing bilirubin output

Oxidation of urobilinogen in the sample

Most of the urobilinogen formed by bacteria in the colon is:

Oxidized to stercobilin in feces

Reabsorbed and lost in the urine

Returned to the liver as bilirubin

Even at high plasma levels, unconjugated bilirubin does not appear in urine. Why?

It is bound to albumin and not filtered

The kidney converts it to urobilinogen

It is broken down by light in the bladder

It is fully reabsorbed by the tubules

Phototherapy lowers unconjugated bilirubin in a jaundiced newborn mainly by:

Making isomers excreted without conjugation

Increasing albumin binding of bilirubin

Inducing UGT1A1 enzyme activity in the liver

Oxidizing bilirubin back to biliverdin

A 4-day-old preterm infant has unconjugated bilirubin 13 mg/dL (222 µmol/L) and albumin 2.5 g/dL (25 g/L). A sulfonamide antibiotic is being considered. Why does this drug raise the risk of bilirubin brain injury?

It displaces bilirubin from albumin

It stops bacteria forming urobilinogen

It blocks bilirubin secretion into bile

It binds bilirubin covalently to albumin

Heme derivatives: hemoglobin, bilirubin, porphyrins - physiology

11 ASCP MLS practice questions on Heme derivatives: hemoglobin, bilirubin, porphyrins - physiology, part of Chemistry. 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.

Question 1 · Hard
A 24-year-old man with hereditary spherocytosis is seen on day 9 of oral broad-spectrum antibiotics for a deep wound infection. Hemoglobin 10.6 g/dL [106 g/L], reticulocytes 8.5%, total bilirubin 3.9 mg/dL [67 µmol/L], direct bilirubin 0.3 mg/dL [5 µmol/L], ALP and ALT normal; these are close to his usual values. A freshly voided urine, protected from light and tested within 15 minutes, shows bilirubin negative and urobilinogen 0.2 EU/dL (the lowest pad block). Two months ago, with similar blood results, his urine urobilinogen was 4.0 EU/dL. What best explains the change?
1. ABile duct obstruction stopping bile flow
2. BLess hemolysis reducing bilirubin output
3. COxidation of urobilinogen in the sample
4. DFewer colonic bacteria to convert bilirubin
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Answer: D · Fewer colonic bacteria to convert bilirubin
Conjugated bilirubin excreted in bile is reduced to urobilinogen by bacteria in the colon; part is reabsorbed and a small amount appears in urine. In hemolysis more bilirubin reaches the gut, so urine urobilinogen is usually raised, as his earlier result shows. Broad-spectrum antibiotics suppress the colonic flora, so less urobilinogen is formed even though bilirubin output is unchanged. A normal direct bilirubin, ALP and urine bilirubin exclude obstruction, and prompt testing away from light excludes oxidation to urobilin. The same lack of gut flora explains the low urobilinogen of newborns.
- A: Direct bilirubin, ALP and urine bilirubin are normal, so bile is reaching the gut.
- B: Reticulocytes and unconjugated bilirubin are unchanged from his usual values.
- C: The urine was fresh, protected from light and tested at once.
Question 2 · Medium
Most of the urobilinogen formed by bacteria in the colon is:
1. AReabsorbed and lost in the urine
2. BReturned to the liver as bilirubin
3. COxidized to stercobilin in feces
4. DReconjugated in the gut wall
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Answer: C · Oxidized to stercobilin in feces
Gut bacteria deconjugate bilirubin and reduce it to colorless urobilinogens. Most are oxidized to orange-brown stercobilin (urobilin), which colors the stool. About 10 to 20% is reabsorbed; most of that is taken up and re-excreted by the liver, and only a small amount is filtered into urine. Absent bile flow therefore gives pale stools and low urobilinogen.
- A: Only a small part is reabsorbed, and most of that is re-excreted in bile.
- B: Reabsorbed urobilinogen is re-excreted by the liver, not turned back into bilirubin.
- D: Urobilinogen is not reconjugated in the gut wall.
Question 3 · Medium
Even at high plasma levels, unconjugated bilirubin does not appear in urine. Why?
1. AIt is bound to albumin and not filtered
2. BThe kidney converts it to urobilinogen
3. CIt is broken down by light in the bladder
4. DIt is fully reabsorbed by the tubules
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Answer: A · It is bound to albumin and not filtered
Unconjugated bilirubin is not water-soluble and circulates tightly bound to albumin, which is too large to pass the glomerulus. Conjugated bilirubin is water-soluble and only loosely bound, so it is filtered once its plasma level rises. A positive urine bilirubin therefore points to raised conjugated bilirubin. Jaundice from unconjugated bilirubin alone was once called acholuric jaundice.
- B: Urobilinogen is formed by gut bacteria, not by the kidney.
- C: Light breaks down bilirubin in samples, but that is not why urine is negative.
- D: Bound unconjugated bilirubin is not filtered, so there is nothing to reabsorb.
Question 4 · Medium
Phototherapy lowers unconjugated bilirubin in a jaundiced newborn mainly by:
1. AMaking isomers excreted without conjugation
2. BIncreasing albumin binding of bilirubin
3. CInducing UGT1A1 enzyme activity in the liver
4. DOxidizing bilirubin back to biliverdin
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Answer: A · Making isomers excreted without conjugation
Blue-green light of about 460 to 490 nm is absorbed by bilirubin in the skin and capillaries. It converts bilirubin into configurational isomers and the structural isomer lumirubin, which are more water-soluble. These can be excreted in bile, and lumirubin also in urine, without conjugation. This bypasses the newborn's immature UGT1A1.
- B: Light does not increase albumin binding.
- C: Drugs such as phenobarbital, not light, induce UGT1A1.
- D: The main effect is a change in bilirubin's shape, not oxidation back to biliverdin.
Question 5 · Hard
A 4-day-old preterm infant has unconjugated bilirubin 13 mg/dL (222 µmol/L) and albumin 2.5 g/dL (25 g/L). A sulfonamide antibiotic is being considered. Why does this drug raise the risk of bilirubin brain injury?
1. AIt stops bacteria forming urobilinogen
2. BIt blocks bilirubin secretion into bile
3. CIt displaces bilirubin from albumin
4. DIt binds bilirubin covalently to albumin
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Answer: C · It displaces bilirubin from albumin
Only unbound (free) unconjugated bilirubin crosses the blood-brain barrier and damages the basal ganglia (kernicterus). Sulfonamides compete with bilirubin for albumin binding sites, so free bilirubin rises even if the total does not change. Low albumin, acidosis and prematurity raise the risk further. This is why total bilirubin alone does not fully reflect the danger in a sick newborn.
- A: Urobilinogen formation in the gut does not affect bilirubin entry into the brain.
- B: The danger comes from free bilirubin in the blood, not from blocked bile secretion.
- D: Covalent (delta) binding forms in long-standing conjugated hyperbilirubinemia and is not caused by the drug.
Question 6 · Easy
In the hepatocyte, bilirubin is made water-soluble by conjugation with:
1. AGlutathione
2. BGlucuronic acid
3. CSulfate
4. DGlycine
Answer and explanation in LearnFree account
Question 7 · Hard
A healthy 22-year-old becomes mildly jaundiced after two days of fasting. Total bilirubin is 3.2 mg/dL (55 µmol/L) and direct bilirubin 0.2 mg/dL (3 µmol/L); hemoglobin, reticulocytes, haptoglobin, ALT and ALP are normal. Which step of bilirubin metabolism is most likely reduced?
1. AReduction to urobilinogen in the gut
2. BSecretion into the bile canaliculi
3. CConjugation by UGT1A1
4. DBreakdown of heme to biliverdin
Answer and explanation in LearnFree account
Question 8 · Easy
Heme oxygenase in macrophages opens the heme ring, releasing iron, biliverdin and:
1. ACarbon dioxide
2. BUrobilinogen
3. CCarbon monoxide
4. DAmmonia
Answer and explanation in LearnFree account
Question 9 · Hard
Five days after an obstructing common bile duct stone is removed, a patient is still jaundiced. Total bilirubin is 6.4 mg/dL (109 µmol/L) and direct bilirubin 5.6 mg/dL (96 µmol/L), but urine bilirubin is now negative and ALP is falling. What best explains these results?
1. ANew hemolysis adding unconjugated bilirubin
2. BDelta bilirubin still in the circulation
3. CLoss of conjugating ability in the liver
4. DOngoing partial blockage of the duct
Answer and explanation in LearnFree account
Question 10 · Easy
Unconjugated bilirubin is insoluble in water. It travels from macrophages in the spleen to the liver bound to:
1. ATransferrin
2. BHemopexin
3. CHaptoglobin
4. DAlbumin
Answer and explanation in LearnFree account
Question 11 · Medium
Which step in the liver's handling of bilirubin is rate-limiting?
1. ASecretion of conjugates into bile
2. BBinding to ligandin in the cytosol
3. CUptake of bilirubin from plasma
4. DConjugation by UGT1A1
Answer and explanation in LearnFree account

Heme derivatives: hemoglobin, bilirubin, porphyrins - physiology

Most of the urobilinogen formed by bacteria in the colon is:

Even at high plasma levels, unconjugated bilirubin does not appear in urine. Why?

Phototherapy lowers unconjugated bilirubin in a jaundiced newborn mainly by:

A 4-day-old preterm infant has unconjugated bilirubin 13 mg/dL (222 µmol/L) and albumin 2.5 g/dL (25 g/L). A sulfonamide antibiotic is being considered. Why does this drug raise the risk of bilirubin brain injury?

In the hepatocyte, bilirubin is made water-soluble by conjugation with:

A healthy 22-year-old becomes mildly jaundiced after two days of fasting. Total bilirubin is 3.2 mg/dL (55 µmol/L) and direct bilirubin 0.2 mg/dL (3 µmol/L); hemoglobin, reticulocytes, haptoglobin, ALT and ALP are normal. Which step of bilirubin metabolism is most likely reduced?

Heme oxygenase in macrophages opens the heme ring, releasing iron, biliverdin and:

Five days after an obstructing common bile duct stone is removed, a patient is still jaundiced. Total bilirubin is 6.4 mg/dL (109 µmol/L) and direct bilirubin 5.6 mg/dL (96 µmol/L), but urine bilirubin is now negative and ALP is falling. What best explains these results?

Unconjugated bilirubin is insoluble in water. It travels from macrophages in the spleen to the liver bound to:

Which step in the liver's handling of bilirubin is rate-limiting?