HSC Exam Practice

Sample response. A prion is an infectious, non-cellular pathogen consisting solely of a misfolded protein (PrP^Sc); it contains no nucleic acid. It causes disease by inducing normal host proteins to adopt the misfolded conformation, leading to neurodegenerative disease (e.g. BSE in cattle, CJD in humans). A virus differs from a prion in that it contains a nucleic acid genome (DNA or RNA) enclosed in a protein coat (capsid); some viruses also have a lipid envelope. Both are non-cellular, but a virus relies on its nucleic acid to direct replication inside a host cell, whereas a prion has no nucleic acid and instead propagates by protein misfolding alone.

Marking notes. 1 mark — defines prion correctly as a misfolded protein with no nucleic acid. 1 mark — defines virus correctly (nucleic acid + capsid; replicates inside host cell). 1 mark — makes an explicit comparison identifying at least one similarity (both non-cellular) and one difference (virus has nucleic acid; prion does not).

Sample response. (a) Wheat stem rust is caused by a fungus (Puccinia graminis). Justification: it is a eukaryotic organism with a cell wall containing chitin, and it spreads via spores. (b) Malaria is caused by a protozoan (Plasmodium falciparum). Justification: it is a unicellular eukaryote with a complex multi-host life cycle (mosquito vector + human host). (c) Potato spindle tuber disease is caused by a viroid (PSTVd). Justification: it consists only of single-stranded circular RNA with no protein coat; it is the smallest known class of pathogen and infects plants only.

Marking notes. 1 mark per pathogen: correct type + one valid structural/biological justification. Justification alone without type name, or type name alone, scores 0 for that item.

Sample response. Penicillin works by inhibiting the cross-linking of peptidoglycan chains in the bacterial cell wall. Influenza is a non-cellular pathogen (a virus) — it has no cell wall and therefore contains no peptidoglycan. Because the molecular target of penicillin is entirely absent in influenza virus, the drug has no mechanism by which to inhibit viral replication or damage the virus. Using penicillin for influenza will not reduce viral load, shorten illness, or prevent transmission; it will, however, contribute to antibiotic resistance in bacteria already present in the patient.

Marking notes. 1 mark — names peptidoglycan cell wall as penicillin’s target. 1 mark — correctly states that influenza virus (as a non-cellular pathogen) has no cell wall / no peptidoglycan. 1 mark — draws the causal conclusion that the absence of the target means penicillin has no mechanism of action against the virus.

Sample response. Hookworm larvae (Ancylostoma duodenale) actively burrow through intact human skin using proteolytic enzymes that break down proteins in the skin barrier, combined with physical movement through soft tissue. This transcutaneous penetration allows direct entry without ingestion. Influenza A virus uses a surface glycoprotein called haemagglutinin that binds specifically to sialic acid receptors on the surface of respiratory epithelial cells; once bound, the cell engulfs the virus by endocytosis, allowing the viral genome to enter and replicate. Comparison: both mechanisms target a specific biological interface (skin vs respiratory epithelium) and use a biological process to cross it. However, hookworm larvae use enzymatic degradation of host tissue as a physical barrier-breaching mechanism, while influenza uses molecular receptor–ligand binding to trigger the host cell to actively internalise the virus — a fundamentally different entry strategy that exploits normal host cell endocytic processes.

Marking notes. 1 mark — correctly describes hookworm entry (skin penetration using proteolytic enzymes / physical burrowing). 1 mark — correctly describes influenza entry (haemagglutinin binds sialic acid receptor on respiratory epithelium; endocytosis). 1 mark — identifies one similarity in the comparison (both target a specific tissue interface). 1 mark — identifies one difference in the comparison (enzymatic/mechanical barrier breach vs receptor-mediated endocytosis).

Sample response. Adaptation 1: Plasmodium develops in the salivary glands of the female Anopheles mosquito (as sporozoites). When the mosquito takes a blood meal, sporozoites are injected directly into the human bloodstream, bypassing the host’s skin barrier entirely. This vector-mediated transmission means Plasmodium does not need to survive in the external environment or penetrate skin on its own — the mosquito does both. Adaptation 2: Plasmodium infects and replicates within red blood cells and liver cells. During the liver stage it can lie dormant for extended periods, allowing the host to remain asymptomatic and mobile, continuing to feed mosquitoes during this infectious window. The dormant liver stage thus extends the transmission period and increases the probability of a feeding mosquito becoming a new vector.

Marking notes. 1 mark + 1 mark for Adaptation 1: correctly identifies sporozoite development in mosquito salivary glands [1] and explains how injection during blood meal bypasses the host skin barrier [1]. 1 mark + 1 mark for Adaptation 2: correctly identifies a second adaptation (liver dormancy / long asymptomatic period / gametocyte production in blood) [1] and explains the transmission mechanism it enables [1].

Sample response. TMV has an extremely stable rod-shaped protein coat (capsid) with no lipid envelope. The tightly interlocked capsid protein subunits protect the RNA genome from desiccation, UV radiation and moderate temperatures for years on contaminated surfaces and tools. HIV is an enveloped retrovirus — it has a lipid bilayer derived from the host cell membrane surrounding the protein capsid. Lipid bilayers are extremely sensitive to drying, changes in osmolarity, and exposure to air; without a cell to maintain the membrane, the envelope rapidly disrupts, which destroys the virus’s ability to infect a new cell. The structural difference (stable naked capsid in TMV vs fragile lipid envelope in HIV) directly accounts for the dramatically different environmental survival times of the two viruses.

Marking notes. 1 mark — identifies TMV’s stable naked capsid (no envelope) as the structural basis of its environmental persistence. 1 mark — identifies HIV’s lipid envelope and explains that it is structurally fragile outside the host cell (disrupts rapidly on drying / exposure to air). 1 mark — makes an explicit structural comparison that links the presence/absence of a lipid envelope to the observed difference in environmental survival.

Sample response (a). The data show a strong positive correlation between Anopheles mosquito density and malaria case incidence across both 12-month cycles. In both years, mosquito density and case incidence peak together in months 2–4 and 14–16 (wet season), and both fall to near-zero in the dry season months. The rise in mosquito density appears to slightly precede the peak in malaria cases by approximately one month, consistent with an incubation period between mosquito bite and confirmed case.

Sample response (b). Anopheles mosquitoes are the obligate vector for Plasmodium vivax. The parasite undergoes part of its life cycle inside the mosquito, developing into infectious sporozoites in the mosquito’s salivary glands. When the female mosquito takes a blood meal from a human host, sporozoites are injected directly into the bloodstream — this is the specific transmission adaptation. Higher mosquito densities during the wet season mean more blood-meal contacts per person per unit time, directly increasing the rate of sporozoite injection and therefore the rate of new infections. The ~1-month lag between mosquito peak and case peak in the graph is consistent with the incubation period of P. vivax in the human liver (pre-erythrocytic stage) before symptoms develop and cases are confirmed.

Marking notes (a). 1 mark — identifies a positive correlation / parallel trends between mosquito density and malaria incidence. 1 mark — supports with specific reference to seasonal peaks (wet vs dry season) or quotes approximate months from the graph. (b) 1 mark — identifies the Anopheles mosquito as the transmission vector. 1 mark — describes the specific transmission adaptation (sporozoites in salivary glands; injected during blood meal). 1 mark — links higher mosquito density to higher injection rate and therefore higher case incidence. 1 mark — accounts for the lag between mosquito peak and case peak using an incubation / pre-erythrocytic stage argument.

Sample response. HIV is a retrovirus — a non-cellular pathogen containing RNA and the enzyme reverse transcriptase, enclosed in a protein capsid and lipid envelope. It infects CD4+ T helper cells by binding its surface glycoprotein (gp120) to the CD4 receptor, then inserts a DNA copy of its genome into the host’s chromosomes using integrase. Antiretroviral drugs (e.g. reverse transcriptase inhibitors, integrase inhibitors) target these specific viral enzymes; no antibiotic or anthelmintic has any mechanism against these targets.

Influenza A is also a non-cellular pathogen (virus) with a segmented RNA genome in a lipid envelope. Its surface haemagglutinin binds sialic acid receptors on respiratory epithelial cells; neuraminidase on the envelope cleaves sialic acid to release new viral particles. Oseltamivir (Tamiflu) is a neuraminidase inhibitor — it specifically blocks neuraminidase, trapping new viral particles on the infected cell and reducing viral spread. An antiretroviral targeting HIV’s reverse transcriptase would have no effect on influenza’s neuraminidase; an anthelmintic targeting tubulin in worms would have no effect on any protein in either virus.

Taenia solium (tapeworm) is a macroorganism — a multicellular animal (helminth). It attaches to the intestinal wall using hooks and suckers and absorbs digested nutrients through its body wall. Mebendazole disrupts tubulin polymerisation in the worm’s cells, preventing cell division and glucose uptake. This mechanism is entirely irrelevant to any virus: viruses have no cells, no tubulin and no glucose-uptake pathway of their own.

These three examples confirm that accurate pathogen classification is not only the most important first step in managing an infectious disease — it is a necessary precondition for choosing any effective intervention. The alternative is reaching for a drug that has no molecular target in the actual pathogen: prescribing antibiotics for HIV or mebendazole for influenza would not reduce viral load by a single particle. Classification also avoids active harm: prescribing antibiotics for a viral infection selects for antibiotic resistance in the patient’s existing bacterial microbiome. The claim is therefore well-founded: misclassification does not merely delay treatment, it guarantees treatment failure and may worsen the patient’s broader infectious disease risk.

Marking notes. 1 mark — classifies HIV correctly (retrovirus) and names at least one relevant structural feature (reverse transcriptase / integrase / gp120) linked to its specific treatment. 1 mark — classifies influenza correctly (RNA virus) and names at least one relevant structural feature (neuraminidase / haemagglutinin) linked to its specific treatment (oseltamivir). 1 mark — classifies Taenia solium correctly (helminth / macroorganism) and names at least one relevant biological feature (tubulin / multicellular animal) linked to its specific treatment (mebendazole). 1 mark — makes an explicit cross-comparison showing that the drug effective against one pathogen is ineffective against at least one other, with a structural reason. 1 mark — evaluates the claim: classification is necessary (not merely useful) because without it there is no basis for matching drug mechanism to pathogen biology. 1 mark — extends the evaluation to include active harm from misclassification (antibiotic resistance / treatment delay / microbiome disruption) or makes a nuanced judgement that classification is necessary but not sufficient (correct diagnosis also requires knowing the specific strain, resistance profile, etc.).