TestAS · Medicine
Biomolecules, enzymes, metabolism, acids and buffers and concentrations.
testas.de lists this topic for the Medicine module. In the test it comes as testlets: a short text, table or formula, then 4 to 8 questions you answer from it.
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Full exam-length testlets from the Kollego course, generated the same way as our mocks. Answer from the material, then open each answer to check.
Bilirubin and the causes of jaundice
Red blood cells live for about 120 days. When old cells are broken down in the spleen, the haem part of haemoglobin is converted into bilirubin. This bilirubin is not water-soluble; it travels in the blood bound to albumin and cannot be excreted in the urine. It is called unconjugated (or indirect) bilirubin.
The liver takes up unconjugated bilirubin and attaches glucuronic acid to it. This conjugated (direct) bilirubin is water-soluble and is excreted into the bile, which flows through the bile ducts into the small intestine. In the gut, bacteria convert it into urobilinogen. Most of the urobilinogen is turned into pigments that give stool its brown colour; a small part is absorbed into the blood and excreted in the urine.
If the bilirubin level in the blood rises above about 2 to 3 mg/dL, the whites of the eyes and the skin turn yellow (jaundice). The cause can be located by asking where the flow of bilirubin is disturbed. If red blood cells are destroyed faster than normal (haemolysis, a pre-hepatic cause), more bilirubin is produced than the liver can process: unconjugated bilirubin rises, but none appears in the urine, and more urobilinogen is formed. If the liver cells are damaged (hepatic cause, as in hepatitis), both forms can rise. If the flow of bile is blocked, for example by a gallstone in the common bile duct or a tumour of the pancreas (post-hepatic cause), conjugated bilirubin flows back into the blood. Because it is water-soluble, it appears in the urine and makes it dark; little bilirubin reaches the gut, so the stool becomes pale, and urobilinogen in the urine disappears. Bile acids also flow back into the blood and can cause itching.
Many newborns develop mild jaundice in the first days of life: they break down many red blood cells and their liver enzymes are still immature. Very high levels of unconjugated bilirubin can damage the newborn brain and are treated with blue light, which converts bilirubin into a form that can be excreted.
| Patient | Unconjugated bilirubin | Conjugated bilirubin | Bilirubin in urine | Urobilinogen in urine | Stool |
|---|---|---|---|---|---|
| W | Raised | Normal | Absent | Raised | Brown |
| X | Normal | Strongly raised | Present, dark urine | Absent | Pale |
| Y | Raised | Raised | Present | Present | Brown |
| Z | Raised | Normal | Absent | Normal | Brown |
1. Which patient most likely has a gallstone blocking the common bile duct?
C: Patient X
A blocked bile flow raises conjugated bilirubin, which appears in the urine (dark), while little reaches the gut: pale stool and no urobilinogen in the urine. This is X's pattern.
2. Which patient's findings fit haemolysis best?
C: Patient W
Haemolysis raises unconjugated bilirubin, which cannot enter the urine, and more urobilinogen is formed. W has exactly this: raised unconjugated bilirubin, no urinary bilirubin, raised urobilinogen.
3. Why does patient W have no bilirubin in the urine despite jaundice?
D: His raised bilirubin is unconjugated and bound to albumin
The text says unconjugated bilirubin is not water-soluble, travels bound to albumin and "cannot be excreted in the urine".
4. Patient Y has raised unconjugated and conjugated bilirubin with bilirubin in the urine. What is most likely?
B: Damage to the liver cells, such as hepatitis
The text says in hepatic causes "both forms can rise". The stool is still brown and urobilinogen present, so bile still reaches the gut, which argues against a complete block.
5. Patient Z has only a mildly raised unconjugated bilirubin, normal urobilinogen and no signs of haemolysis. Which statement is justified by the text?
C: The text does not allow a clear diagnosis for Z
Z's pattern is neither the post-hepatic one (normal conjugated bilirubin, brown stool) nor clearly haemolysis (urobilinogen not raised). The text gives no cause that matches exactly, so no clear diagnosis is possible from it.
6. Why does a patient with a tumour of the pancreas blocking the bile duct often complain of itching?
A: Bile acids flow back into the blood
The text says when bile flow is blocked, "Bile acids also flow back into the blood and can cause itching".
7. Why are newborns with high bilirubin treated with blue light?
A: It converts bilirubin into a form that can be excreted
The text says blue light "converts bilirubin into a form that can be excreted".
How the body breaks down alcohol
Ethanol, the alcohol in drinks, is absorbed rapidly from the stomach and small intestine and distributes in the body water. Because women on average have less body water per kilogram than men, the same amount of alcohol produces a higher blood alcohol concentration in a woman of the same weight. More than 90% of the alcohol is broken down in the liver; only a small part is exhaled or excreted unchanged.
The breakdown occurs in two steps. First, the enzyme alcohol dehydrogenase (ADH) oxidises ethanol to acetaldehyde, a toxic substance that causes flushing, headache and nausea. Second, aldehyde dehydrogenase (ALDH) oxidises acetaldehyde to acetate, which is used for energy. In both steps, NAD+ is reduced to NADH. Unlike most drugs, ethanol is eliminated at an almost constant rate, about 0.1 to 0.15 per mille of blood alcohol per hour in adults, because the enzyme is saturated even at moderate concentrations (zero-order kinetics). Coffee, cold showers or exercise do not speed this up. A small part of the alcohol is also oxidised by a second enzyme system in the liver, which becomes more active in people who drink regularly and partly explains why they tolerate alcohol better.
Hours until sober ≈ blood alcohol (per mille) ÷ elimination rate (per mille per hour)
The large amount of NADH produced has metabolic consequences. It inhibits gluconeogenesis, the formation of new glucose in the liver, so that people who drink heavily without eating can develop dangerously low blood sugar, especially children. It also favours the synthesis of fat in the liver, which is why fatty liver is an early consequence of regular heavy drinking. Over the years, fatty liver can progress to inflammation and finally to cirrhosis.
Many people of East Asian origin carry a variant of the ALDH2 gene that produces an almost inactive enzyme. After drinking, acetaldehyde accumulates in their blood and causes marked facial flushing and a racing heart; carriers of this variant drink less alcohol on average. The drug disulfiram, used in the treatment of alcohol dependence, blocks ALDH in the same way, so that drinking even a small amount becomes very unpleasant. Methanol, found in illegally produced spirits, is broken down by the same enzymes to formaldehyde and formic acid, which damage the optic nerve and can cause blindness; it is treated with fomepizole or with ethanol itself, which compete for alcohol dehydrogenase.
1. A man has a blood alcohol of 1.5 per mille at midnight. If he eliminates 0.15 per mille per hour, when will it be close to zero?
B: At about 10:00 in the morning
Hours until sober ≈ 1.5 ÷ 0.15 = 10 hours. Midnight plus 10 hours is about 10:00 in the morning.
2. Why does drinking coffee not help to become sober faster?
B: Alcohol is eliminated at a nearly constant rate set by a saturated enzyme
The text says ethanol is eliminated at an almost constant rate because the enzyme is saturated (zero-order kinetics), and "Coffee, cold showers or exercise do not speed this up".
3. A child drinks a glass of spirits at a party and becomes drowsy and confused hours later. Which complication does the text suggest should be checked first?
D: Low blood sugar caused by inhibited gluconeogenesis
The text says the large amount of NADH inhibits gluconeogenesis, so people who drink without eating can develop dangerously low blood sugar, especially children.
4. Why does a person with an inactive ALDH2 variant flush after a small amount of alcohol?
D: Acetaldehyde accumulates because it is not converted to acetate
The text says with an almost inactive ALDH, acetaldehyde accumulates in the blood and causes marked facial flushing and a racing heart.
5. Why is a patient with methanol poisoning given ethanol?
C: Ethanol competes for alcohol dehydrogenase, so less methanol becomes toxic acid
The text says methanol is broken down by the same enzymes to toxic products, and ethanol or fomepizole compete for alcohol dehydrogenase.
6. A woman and a man of the same weight drink the same amount of alcohol. Why is the woman's blood alcohol usually higher?
D: On average she has less body water per kilogram
The text says alcohol distributes in the body water, and women on average have less body water per kilogram, so the same amount gives a higher concentration.
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