HSC Exam Practice

Sample response. Malaria is caused by Plasmodium — a eukaryotic protozoan parasite (not a virus or bacterium). The species responsible for the most lethal form of human malaria is Plasmodium falciparum.

Marking notes. 1 mark for correctly identifying Plasmodium as a eukaryotic protozoan (or "parasite" — must not say "virus" or "bacterium"); 1 mark for naming P. falciparum as the most lethal species.

Sample response. Anopheles (female) transmits malaria (Plasmodium); it is a nocturnal biter that rests indoors and is found predominantly in sub-Saharan Africa and other tropical regions. Aedes aegypti transmits dengue virus (and also Zika and chikungunya); it is an urban, daytime biter that breeds in small standing-water containers such as tyres, pots, and gutters.

Marking notes. 1 mark for correctly pairing each mosquito with its disease; 1 mark for a correct behavioural difference (nocturnal vs diurnal, indoor-resting vs container-breeding, or equivalent); 1 mark for identifying the urban/domestic habitat of Aedes aegypti or the predominantly rural/savanna habitat of Anopheles (any valid third distinction).

Sample response. Stage 1 — Liver stage: sporozoites injected by the mosquito migrate to the liver and invade hepatocytes, multiplying asexually over 7–14 days to produce merozoites (asymptomatic). Stage 2 — Blood stage: merozoites released into the bloodstream invade red blood cells (RBCs), multiply, cause RBCs to burst (producing fever and anaemia), and produce gametocytes. Stage 3 — Mosquito pickup stage: gametocytes are taken up by a feeding Anopheles mosquito, where sexual reproduction occurs in the mosquito gut, producing sporozoites that migrate to the salivary glands to restart the cycle.

Marking notes. 1 mark per stage: location + what happens to parasite. Accept any three correctly described stages from the full life cycle (mosquito injection + liver + blood; or liver + blood + mosquito sexual stage). Each stage must name both where it occurs AND what happens to the parasite for the mark.

Sample response. Drug resistance in Plasmodium falciparum is the product of natural selection operating on the existing genetic variation in the parasite population. Individual parasites within a large population vary in their sensitivity to artemisinin — some carry mutations (e.g. in the Kelch13 propeller domain) that reduce drug binding. When ACT is administered, this creates a selection pressure: sensitive parasites are killed; resistant parasites survive and continue to reproduce. Because the resistance-conferring alleles are heritable (passed to daughter cells during asexual replication in red blood cells), resistant genotypes increase in frequency over successive parasite generations. Over time — with repeated ACT exposure across a region — the population becomes dominated by resistant parasites. This process has been observed in Southeast Asia and is now emerging in East Africa.

Marking notes. 1 mark — identifies variation in drug sensitivity within the Plasmodium population as the necessary starting condition; 1 mark — explains selection pressure (ACT kills sensitive parasites; resistant parasites survive and reproduce preferentially); 1 mark — explains heritability (resistance alleles passed to offspring during asexual replication, increasing frequency over generations); 1 mark — applies this mechanism to the specific malaria-ACT context with a named example (ACT / artemisinin / Kelch13 mutations / Southeast Asia) or a clearly general conclusion about the population-level outcome.

Sample response. Antibody-dependent enhancement (ADE) is the process where pre-existing antibodies from a previous dengue serotype infection bind to — but cannot neutralise — a second dengue serotype. Instead, the antibody–virus complex is recognised by Fc receptors on immune cells (monocytes/macrophages), facilitating viral entry and amplifying intracellular replication. The result is a more severe infection — potentially dengue haemorrhagic fever — compared to infection in a naive individual. ADE makes dengue vaccination difficult because a vaccine must provide strong, balanced protection against all four DENV serotypes simultaneously. If it confers immunity to some serotypes but not others, vaccinated individuals may experience ADE when exposed to an unprotected serotype — as occurred with Dengvaxia in seronegative Filipino schoolchildren. Malaria does not have this multi-serotype immune paradox: the RTS,S and R21 vaccines target the circumsporozoite protein of a single parasite and do not risk making recipients worse off if partial protection exists.

Marking notes. 1 mark — correctly defines ADE (pre-existing antibodies facilitate entry of second serotype rather than neutralising it); 1 mark — explains the mechanism (Fc receptor binding amplifies infection, causing more severe disease); 1 mark — explains why ADE complicates dengue vaccination (partial serotype coverage leaves vaccinated individuals at risk of ADE from unprotected serotypes, not just lack of protection); 1 mark — contrasts with malaria vaccination (no serotype paradox / single protein target / vaccines do not risk ADE, or equivalent valid comparison).

Sample response. Both RTS,S (Mosquirix) and R21/Matrix-M target the circumsporozoite surface protein of Plasmodium falciparum, stimulating antibodies that block sporozoite invasion of liver cells — targeting the liver stage of the life cycle. RTS,S showed approximately 36% efficacy against clinical malaria in Phase III trials in young children and requires a four-dose schedule. R21/Matrix-M contains a higher antigen density and showed approximately 75–80% efficacy in trials; it was approved by the WHO in 2023 and represents the most effective malaria vaccine to date. Both still require booster doses and produce waning immunity.

Marking notes. 1 mark for correctly identifying that both vaccines target the circumsporozoite protein / liver stage; 1 mark for stating approximate efficacy figures for both (RTS,S ~36%; R21 ~75–80%); 1 mark for identifying the mechanism of action correctly (antibodies blocking sporozoite invasion of hepatocytes / liver-stage entry). Accept any equivalent wording for full marks.

Part (a) — Calculation (1 mark). Percentage reduction = (18.2 − 2.5) / 18.2 × 100 = 15.7 / 18.2 × 100 = 86.3%. Working must be shown. Accept 86% (consistent with the table's stated value) for 1 mark if working is shown.

Part (b) — Biological explanation (2 marks). The hospitalisation reduction exceeds the incidence reduction, suggesting that Wolbachia infection of the mosquito may also reduce the inoculating viral dose delivered per bite — because the mosquito's reduced vector competence lowers viral titres in the salivary glands as well as transmission probability [1]. Consequently, individuals who do become infected may receive a smaller initial viral load, resulting in milder illness that does not require hospitalisation [1]. Accept also: reduced ADE risk — with lower overall dengue incidence, fewer people in the community have prior serotype exposure creating the ADE susceptibility that leads to severe disease requiring hospitalisation; this compounds the incidence effect to produce a larger hospitalisation reduction.

Part (c) — Mechanistic difference and significance (3 marks). ITNs reduce malaria burden by directly killing or repelling the Anopheles vector before it can deliver sporozoites — a mechanism that exerts selection pressure on Anopheles populations, favouring pyrethroid-resistant mosquitoes over generations [1]. Wolbachia release reduces dengue burden by modifying the mosquito's internal biology (reducing vector competence) without killing the mosquito — there is no lethal selection pressure on the mosquito, so there is no mechanism analogous to insecticide resistance through which the mosquito population can evolve to escape the Wolbachia effect [1]. This is significant because ITN effectiveness is being progressively eroded by pyrethroid resistance, while Wolbachia-based control is theoretically more durable — its effectiveness does not depend on killing the mosquito and therefore cannot be undermined by survival-of-the-fittest selection in the same way [1].

Sample response. The claim that disease persistence is primarily a failure of biological knowledge misrepresents the situation: both malaria and dengue persist despite the availability of effective control tools, largely because of delivery, funding, and political will failures — though biological challenges (resistance and immune complexity) are compounding factors, not the primary cause.

For malaria, the existence of effective tools is well established. Insecticide-treated bed nets (ITNs) reduce child malaria mortality by approximately 50–60% in high-use areas; artemisinin combination therapy (ACT) cures clinical malaria in the vast majority of cases; the R21/Matrix-M vaccine achieves approximately 75–80% efficacy against clinical malaria — the highest of any malaria vaccine. Between 2000 and 2015, scaled deployment of ITNs and ACT reduced malaria deaths in sub-Saharan Africa by approximately 58%, demonstrating clearly that these tools work when delivered at scale. The persistence of approximately 608,000 annual deaths is not because we lack effective tools — it is because 35% of sub-Saharan African households still lack ITN coverage, ACT is often unavailable in remote areas, and the R21 vaccine faces cold-chain and infrastructure delivery constraints in the world's poorest regions. These are delivery and funding failures, not knowledge gaps.

For dengue, the situation is more mixed. The ADE phenomenon — where antibodies from a prior dengue serotype infection paradoxically amplify a second serotype infection — creates a genuine biological complexity that has eluded a universally safe and effective vaccine. The Dengvaxia disaster in the Philippines, where seronegative children experienced increased severe dengue after vaccination, demonstrates that incomplete biological understanding of ADE was a contributing factor. The Wolbachia release program (77% dengue incidence reduction in Yogyakarta) shows that effective control is possible, but it is not yet deployed globally. Here, biological complexity (four-serotype ADE challenge for vaccines) and delivery failure (Wolbachia not yet scaled globally) are both real barriers — but the persistence of dengue as a ~390-million-infection-per-year disease is substantially driven by insufficient investment in the Wolbachia or breeding-site control programs that do work.

Biological barriers — drug resistance in Plasmodium, insecticide resistance in Anopheles, and the ADE challenge in dengue — are genuine and scientifically important. They demonstrate that existing tools are not indefinitely durable and that continued innovation is necessary. However, they are compounding factors, not primary causes: without resistance, the tools that already exist would substantially reduce disease burden if deployed equitably. The lesson of Australia — which eliminated local malaria transmission in the early 20th century through drainage, DDT, and treatment — is instructive: elimination is biologically feasible where the infrastructure, funding, and political sustained effort exist.

Overall, the claim is rejected: the persistence of malaria and dengue is primarily a failure of delivery, funding, and political will to ensure equitable access to tools that already work, compounded by biological resistance challenges that make the problem harder but not intractable.

Marking criteria (8 marks):

• 1 mark — Correctly states that effective tools already exist for malaria (ITNs, ACT, R21 — at least two named with efficacy or mechanism).
• 1 mark — Uses specific data to show that where tools are delivered at scale, they work (e.g. 2000–2015 58% death reduction in sub-Saharan Africa).
• 1 mark — Identifies a delivery / funding / infrastructure barrier that limits tool effectiveness (cold chain, ITN coverage gap, ACT access, or equivalent).
• 1 mark — Names a dengue example and correctly explains the biological complexity (ADE / four serotypes / Dengvaxia) as a genuine biological challenge, not just a delivery failure.
• 1 mark — Explains drug or insecticide resistance as a biological complication using natural selection (selection pressure on Plasmodium or Anopheles producing resistant genotypes).
• 1 mark — Uses the Australian or comparable example to demonstrate that elimination is biologically feasible when structural conditions are met.
• 1 mark — Correctly distinguishes between primary cause (delivery/funding failure) and compounding factor (biological resistance/complexity) — does not present them as equivalent.
• 1 mark — Reaches an explicit, justified evaluative judgement that engages with the exact claim (that it is "primarily" a biological knowledge failure) — accepts or rejects with evidence-based reasoning.