IGCSE · Practice questions · Drugs and antibiotics

Bacteria yes, viruses no.

Six original Cambridge-style questions on drugs and antibiotics: the definition of a drug, treating bacterial infections, viruses, resistant bacteria, data on a resistant strain and the reasons for using antibiotics only when essential.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Drug, bacteria, resistance.

Drug: any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotics: treat bacterial infections; they kill bacteria but do not affect viruses.
Resistance: some bacteria are resistant, which reduces the effectiveness of antibiotics. The mutation is random; the antibiotic selects it.
Extended: use antibiotics only when essential (never for viruses, full course) to limit resistant strains such as MRSA.

Recall

[2 marks]

Define the term drug and name one drug that is used to treat bacterial infections.

Any substance taken into the body that modifies or affects chemical reactions in the body. ✓
An antibiotic, for example penicillin. ✓

Explain

[3 marks]

A student has a sore throat caused by a virus. Explain why a doctor would not prescribe an antibiotic.

Antibiotics kill bacteria (or stop them growing) but do not affect viruses. ✓
So the antibiotic would not cure the infection or shorten it. ✓
Unnecessary use of antibiotics favours resistant bacteria / increases antibiotic resistance. ✓

Recall

[2 marks]

State what is meant by a resistant bacterium and explain why resistant bacteria are a problem for medicine.

A resistant bacterium is not killed (or not stopped from growing) by an antibiotic. ✓
Infections caused by resistant bacteria are hard to treat, because the antibiotic is less effective or does not work. ✓

Data

[4 marks]

A hospital recorded the percentage of Staphylococcus aureus infections that were resistant to the antibiotic methicillin (MRSA).

Year	2000	2004	2008	2012	2016
Infections that were MRSA / %	12	26	38	30	21

(a) Describe the trend in the data. (b) After 2008 the hospital began prescribing antibiotics only when essential. Suggest how this explains the change after 2008.

(a) The percentage rose from 12% in 2000 to a peak of 38% in 2008, then fell to 21% by 2016. ✓
(b) Before 2008, frequent use of antibiotics killed sensitive bacteria and let resistant MRSA survive and multiply. ✓
After 2008 antibiotics were used less often, so there was less selection pressure favouring MRSA. ✓
Sensitive strains were no longer killed so often, so MRSA made up a smaller proportion of infections. ✓

Explain

[4 marks]

Extended. Explain how resistant bacteria such as MRSA become common in a population, and how using antibiotics only when essential limits this.

A random mutation makes a few bacteria resistant to the antibiotic. ✓
When the antibiotic is used, sensitive bacteria are killed but the resistant ones survive. ✓
The survivors reproduce and pass on resistance, so the proportion of resistant bacteria increases with each use. ✓
Using antibiotics only when essential (not for viral infections, completing the course) means fewer occasions on which resistant bacteria are selected, so resistance develops more slowly. ✓

Analysis

[3 marks]

A patient stops taking a course of antibiotics after three days because they feel better. One week later the infection returns and the same antibiotic no longer works. Suggest an explanation.

After three days the antibiotic had killed most of the sensitive bacteria, so the patient felt better, but some bacteria survived. ✓
The survivors included the resistant bacteria, which were not killed by the antibiotic. ✓
These multiplied, so the returning infection was mostly resistant bacteria and the same antibiotic was not effective. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.