Toxicology: kinetics of alcohol elimination: practice question

A Forensic Medicine question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Forensic Medicine · Toxicology: kinetics of alcohol elimination

Sample item

In a drink-driving case, serial blood samples show that the alcohol level falls in an almost straight line with time and not exponentially. What explains this?

AThe kidneys excrete alcohol in proportion to its concentrationBMost alcohol is exhaled unchanged through the lungsCLiver alcohol dehydrogenase is saturated, so elimination is at a fixed rateDAbsorption continues at the same rate for many hours

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Liver alcohol dehydrogenase is saturated, so elimination is at a fixed rate

At intoxicating concentrations the hepatic enzyme alcohol dehydrogenase is saturated, so a fixed amount is metabolised per hour regardless of the blood level. This is zero-order kinetics, which allows back-calculation of the level at the time of an offence. Only small amounts are lost in breath and urine.

Option by option

A. The kidneys excrete alcohol in proportion to its concentration
A concentration-proportional process would be first-order and give an exponential fall.
B. Most alcohol is exhaled unchanged through the lungs
Only a small fraction of alcohol is exhaled or excreted unchanged.
C. Liver alcohol dehydrogenase is saturated, so elimination is at a fixed rate (correct)
Correct. Saturation of the enzyme gives zero-order, constant-rate elimination.
D. Absorption continues at the same rate for many hours
Absorption is completed in a few hours and does not produce the linear fall.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is C, Liver alcohol dehydrogenase is saturated, so elimination is at a fixed rate. At intoxicating concentrations the hepatic enzyme alcohol dehydrogenase is saturated, so a fixed amount is metabolised per hour regardless of the blood level.

Why is "The kidneys excrete alcohol in proportion to its concentration" not the answer?

A concentration-proportional process would be first-order and give an exponential fall.

Why is "Most alcohol is exhaled unchanged through the lungs" not the answer?

Only a small fraction of alcohol is exhaled or excreted unchanged.

Why is "Absorption continues at the same rate for many hours" not the answer?

Absorption is completed in a few hours and does not produce the linear fall.

What concept does this question test?

It tests Toxicology: kinetics of alcohol elimination in Forensic Medicine. This is zero-order kinetics, which allows back-calculation of the level at the time of an offence. Only small amounts are lost in breath and urine.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Toxicology: kinetics of alcohol elimination?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.