Drugs

• Core 15.1.1 Describe a drug as any substance taken into the body that modifies or affects chemical reactions in the body — in that wording, and without adding to it.
• Core 15.1.2 Describe the use of antibiotics for the treatment of bacterial infections: what they are given for, what they do to the bacteria, and what that means for the person.
• Core 15.1.3 State that some bacteria are resistant to antibiotics, which reduces the effectiveness of antibiotics.
• Core 15.1.4 State that antibiotics kill bacteria but do not affect viruses. Why that is true is not part of the statement; section C teaches it as understanding, and it is what a why question is asking for.
• Supplement 15.1.5 Explain how using antibiotics only when essential can limit the development of resistant bacteria such as MRSA — as a chain from frequency of use, through selection, to the rate at which a resistant strain spreads.

The sentence worth keeping. An antibiotic is a medicine that never meets a virus. Everything a virus does happens inside a host cell, using that cell's machinery, and none of it involves the bacterial structures an antibiotic acts on. Hold that picture and error three and error four both become impossible.

• If you wrote capsule, flagellum or pili, you have learnt a bacterium from somewhere other than this syllabus. They are real structures; they are not on the list.
• An antibiotic is a medicine you take. It is not one of the body's defences, and it is not an antibody. Both of those are Topic 10.
• Spelling matters here more than almost anywhere. "Antibodies kill the bacteria in a chest infection" is a different claim from the one you meant, because the word written names a different thing doing a different job. Slow down on antibiotic against antibody — it is worth writing them out side by side once.
• Supporting context — not a Topic 15 statement Alcohol and nicotine are named here only to show how wide the definition is. What either of them does to health is not part of this syllabus and is not asked anywhere in Topic 15.
• Supporting context — not a Topic 15 statement What a virus is made of, and the fact that it can only reproduce inside a host cell, are not numbered Topic 15 requirements — they are why 15.1.4 is true. Learn them as understanding, and answer the statement with the statement.
• The questions below were written for this chapter, and the mark figures beside them are this chapter's own — chosen to match the number of points the answer needs. They are not Cambridge questions or a Cambridge mark scheme.
• One mark, one sentence, and the sentence has to be this one. Adding "and is harmful" or "and is illegal" makes it wrong.
• The first point alone is statement 15.1.4. The second is the reason behind it — not a numbered requirement of Topic 15, but what this question's why is asking for.
• Suggest means the answer is not written out anywhere for this exact scenario — you are applying 15.1.3. Note what the answer does not say: that the patient became resistant, or that the bacteria have become more dangerous.
• Supporting context — not a Topic 15 statement Selection pressure is not syllabus vocabulary: 0610 describes natural selection at 18.3.1 without the phrase, and Topic 15 never uses it. It is used in this chapter because it names, in two words, the thing whose frequency 15.1.5 asks you to reason about. Every answer below can be written without it — "each course of antibiotics favours the resistant bacteria" says the same thing.
• The questions below were written for this chapter, and the mark figures beside them are this chapter's own — chosen to match the number of points the answer needs. They are not Cambridge questions or a Cambridge mark scheme.
• This is Topic 15's own Supplement statement. Notice that the answer ends by connecting frequency of use to rate of spread — an answer that only describes how resistance arises has answered question 1, and therefore 18.3.5, instead.
• This question joins Core 15.1.4 to Supplement 15.1.5 and is the reason section C ends with a forward pointer. The first marking point is Core; the other three are Supplement.
• These numbers are written for this chapter as a teaching example. They are not taken from a published study, and you should not quote them as evidence of anything outside this page.
• Show the division. The correct figure earns the answer mark by itself, but written working is what allows a sound method to still be credited when the arithmetic slips.
• "It fell by 6%" is ambiguous and can be marked wrong. Say 6 percentage points when you mean the difference between two percentages, and about a third when you mean the relative change.
• These three questions and their mark allocations were written for this chapter to show how an answer is built. They are not Cambridge questions and not a Cambridge mark scheme, and the number in brackets is this chapter's own, chosen to match the number of points the answer needs.
• This question is the mechanism, so the statement it tests is Supplement 18.3.5 in Topic 18, not 15.1.5. Step 4 below shows the difference between the two.
• If your version contained the word harmful, illegal or medicine, it was not this definition. Reopen section A.
• Two clauses, and both are in the syllabus statement. You do not need to say how resistance arose — that is Supplement.
• This is the question that catches people who learnt the definition as words rather than as a test. Run the three clauses in order on anything you are given.
• Check the order. If the antibiotic appears before the word random, the sequence is describing something that does not happen. This is 18.3.5's answer, not 15.1.5's — question 2 is 15.1.5's.
• The syllabus word is limit the development of resistance, not prevent it. Writing "stops resistance happening" is a different, and wrong, claim.
• The questions below were written for this chapter, and the mark figures beside them are this chapter's own — chosen to match the number of points the answer needs. They are not Cambridge questions or a Cambridge mark scheme.
• The question is testing whether you can apply the definition to something nobody would call a drug in conversation. Water, sugar and salts do not fit, because the body uses them in its reactions rather than having those reactions modified by them.
• Notice the last point. Testing first is what makes hospital B's use "only when essential", which is the syllabus's own wording — and saying so is what completes the answer rather than half of it.
• Supporting context — not a Topic 15 statement These answers exist to stop a good question becoming a wrong exam answer. None of them is examinable in itself.

• Where the tiering bites in Topic 15. It bites in one place, and it is worth understanding before you read section D. Core 15.1.3 says state: some bacteria are resistant, so antibiotics become less effective. That is a fact, and a Core answer that gives the fact and its consequence for the patient is complete. Supplement 15.1.5 says explain: why using antibiotics only when essential slows resistance developing — and you cannot explain that without the idea that each course of antibiotics favours resistant bacteria over the rest. So the mechanism lives in the Supplement run, section E, and section D stops before it.
• How much detail 15.1.2 wants. The statement says describe the use of antibiotics for the treatment of bacterial infections. It does not name an antibiotic, and it does not ask how one works at molecular level. "A medicine used to treat infections caused by bacteria; it kills the bacteria so the infection is cleared" is a complete answer. If the question offers more marks, extend it with the chain above — the prescribed course, the fall in bacterial numbers, the symptoms stopping — not with biochemistry.
• Step 5 is the statement. Steps 1 to 4 are how you get there, and an answer that stops at step 4 has explained resistance rather than explained restraint — which is a different question. Whatever else you write, the last sentence must connect how often antibiotics are used to how fast resistance develops.
• A full model answer, in the order a marker reads it. "Random mutation means a few bacteria in a population are already resistant. Using an antibiotic kills the bacteria that are not resistant, so the resistant ones survive and reproduce and the proportion of resistant bacteria rises. Each course of antibiotics is one more occasion on which that happens. So using antibiotics only when they are essential reduces the number of those occasions, and a resistant strain such as MRSA becomes common more slowly — which keeps the antibiotic effective for longer."

• Topic 15 has five statements and no practical of its own, so what it can be asked to do is tightly bounded. Two shapes cover it: short recall of one of the four Core statements, and — for Extended candidates — the one explain the topic contains. What follows is what each paper's scope allows from Topic 15, not a prediction of what any particular paper will contain.
• Drug is the topic's one formal definition. Learn it as a sentence you can write without thinking, because under pressure the everyday meaning is what surfaces.
• Antibiotics kill bacteria; they do not affect viruses. Two clauses. Statement 15.1.4 needs both of them, and needs nothing beyond them.
• Extended only. Fewer unnecessary courses, fewer occasions on which resistant bacteria are favoured, slower spread of the resistant strain. Write it as a chain.
• Describe the trend, quote figures with units, calculate the change, and then be careful — a fall in resistance alongside a fall in prescribing is not by itself proof that one caused the other.
• Command word watch. Describe — 15.1.1 and 15.1.2's verb — wants the account, in order, with no reasons attached. State — 15.1.3 and 15.1.4 — wants the fact and nothing more; a mechanism you were not asked for wastes time and can introduce an error. Explain — 15.1.5 — wants the reason, and in this statement the reason is a causal chain with at least three linked steps. Suggest means the situation is unfamiliar and you are applying the chain, not reciting it.