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Q12 - Antibiotic Pharmacokinetics in a Cow

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Digestive & Metabolic Physiology

The Hungarian Grey is a rare breed of cow which was threatened by an epidemic caused by a new bacterial strain. An antibiotic was given to the cows to prevent infection. It has a half-life in cows of 6 hours, and at equilibrium, 25% of the antibiotic circulates in the blood, whilst 75% is found in interstitium.

A healthy cow received a single dose of 4400 mg intravenously at 11am. At 11pm, 10 ml of blood was withdrawn and 5 mg/l of antibiotic was present in this sample.

Certain fruits (e.g., pomegranate) can inhibit metabolic enzymes in the liver. Using the half-life equation

$$t_{1/2} = \dfrac{t}{\log_{1/2}\left(\dfrac{N(t)}{N_0}\right)}$$

calculate the concentration of the antibiotic in blood at 5pm ($N(t)$) after time $t$ has passed, assuming the half-life ($t_{1/2}$) of the drug has doubled due to pomegranate consumption. $N_0$ is the starting concentration.

Consider the effects of a cow’s treatment with strong oral antibiotics.

Q12.1. A cow is given 4400 mg of an antibiotic IV at 11am. It has a 6-hour half-life, and at equilibrium 25% of it circulates in blood (75% in interstitium). At 11pm, a blood sample's concentration is measured at 5 mg/L. Calculate the cow's total blood volume, in litres (L).
Q12.2. Certain fruits (e.g. pomegranate) inhibit liver metabolic enzymes. Assuming pomegranate consumption doubles the antibiotic's half-life (to 12 hours), calculate the blood concentration at 5pm (6 hours after the 11am dose), in mg/ml, to two decimal places.
Q12.3. The cow will eventually develop antibiotic resistance.
Q12.4. The cow will have increased blood clotting times.
Q12.5. The cow will have reduced nutrient absorption.
Q12.6. The cow will have elevated circulating insulin levels.
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key