HSC Exam Practice

Sample response. Penetrance is the proportion of individuals with a disease-causing genotype who actually express the disease phenotype. Cystic fibrosis has near-100% penetrance because the mechanism is direct: two inherited non-functional CFTR alleles produce absent or non-functional CFTR protein in all relevant cells, and the ion-transport failure occurs universally regardless of other genetic factors. BRCA1-associated breast cancer has approximately 70% penetrance because cancer requires a second somatic mutation inactivating the remaining BRCA1 allele in a specific breast cell (two-hit hypothesis). Whether and when that stochastic somatic mutation occurs depends on individual variation in DNA repair, modifier genes, and chance — so approximately 30% of carriers never acquire the second hit during their lifetime.

Marking notes. 1 mark — defines penetrance correctly (proportion of individuals with disease genotype who express the disease). 1 mark — explains CF near-100% penetrance: both alleles non-functional → CFTR always absent → mechanism always occurs. 1 mark — explains BRCA1 ~70% penetrance: requires second somatic hit (two-hit hypothesis); stochastic; 30% never acquire it.

Sample response. An oncogene arises from a gain-of-function mutation in a proto-oncogene — the mutant protein constitutively drives cell division. Oncogene mutations are functionally dominant: a single mutant allele is sufficient to promote uncontrolled division because the aberrant protein is active regardless of the normal allele's product. A tumour suppressor gene normally inhibits cell division or triggers apoptosis. Its mutations are loss-of-function and recessive: both alleles must be inactivated (two-hit hypothesis) before the braking function is lost, because one functional allele is sufficient to maintain cell cycle control.

Marking notes. 1 mark — oncogene: gain-of-function from proto-oncogene; dominant; one allele sufficient. 1 mark — tumour suppressor: loss-of-function; recessive; two alleles must be inactivated (or two-hit hypothesis stated). 1 mark — correctly contrasts both gene types across all three criteria (mutation type, dominance, alleles required).

Sample response. Iodine is required for synthesis of thyroid hormones T3 and T4. When dietary iodine is insufficient, the thyroid cannot produce adequate T3/T4. Falling T3/T4 levels remove negative feedback on the anterior pituitary, causing it to secrete elevated TSH (thyroid-stimulating hormone). Chronically elevated TSH stimulates thyroid cell proliferation and hyperplasia, causing compensatory enlargement of the gland — goitre.

Marking notes. 1 mark — iodine deficiency prevents adequate T3/T4 synthesis. 1 mark — falling T3/T4 removes negative feedback → elevated TSH from anterior pituitary. 1 mark — elevated TSH drives thyroid cell proliferation → gland enlarges (goitre).

Sample response. Type 2 diabetes is a multifactorial disease. Three contributing risk-factor categories: (1) Genetic — inherited variants in genes such as TCF7L2 and KCNJ11 impair beta-cell insulin secretion and reduce peripheral insulin sensitivity, predisposing individuals to insulin resistance. (2) Nutritional — chronic consumption of excess refined carbohydrates and saturated fats promotes obesity, hyperinsulinaemia, and progressive insulin resistance in muscle, liver, and adipose tissue. (3) Environmental/socioeconomic — night-shift work disrupts circadian rhythms, impairing insulin secretion; low income limits access to nutritious food and safe exercise environments, increasing disease risk independently of individual choices.

Marking notes. 1 mark — identifies multifactorial as the disease category. 1 mark per risk-factor category correctly named and explained with a specific factor (3 categories required, max 3 marks).

Sample response. Metastasis is the process by which malignant tumour cells detach from the primary tumour, invade surrounding tissue and blood or lymphatic vessels (intravasation), survive in circulation, exit at distant sites (extravasation), and establish secondary tumours in organs such as the liver, lungs, or brain. Metastasis is the leading cause of cancer mortality because secondary tumours disrupt the function of multiple vital organs simultaneously, making surgical removal impossible and requiring systemic treatment. The primary tumour alone is often amenable to surgery or localised radiotherapy; it is the systemic dissemination that makes the disease fatal.

Marking notes. 1 mark — defines metastasis correctly (spread from primary tumour via blood/lymph to establish secondary tumours at distant organs). 1 mark — explains why metastasis is more lethal: multiple vital organ disruption; surgical removal impossible. 1 mark — contrasts primary tumour (often treatable locally) with metastatic disease (systemic; untreatable by localised means).

Sample response. Type 1 diabetes arises from autoimmune destruction of pancreatic beta cells (driven by inherited HLA susceptibility variants). The disrupted cellular component is the beta cell itself — all insulin-secreting capacity is permanently lost. Treatment must therefore replace insulin exogenously (daily injections or an insulin pump); no lifestyle modification can restore destroyed beta cells. Type 2 diabetes arises from insulin resistance in peripheral tissues (disrupted insulin receptor signalling in muscle, liver, and adipose tissue) combined with eventual beta-cell exhaustion. Because some beta-cell function and insulin secretion are retained early in the disease, treatment initially targets insulin resistance: metformin reduces hepatic glucose output; lifestyle modification (exercise increases GLUT4 translocation independently of insulin; dietary changes reduce hyperglycaemia). Exogenous insulin is added only when beta-cell reserve is insufficient to compensate. The fundamental difference is that T1D has no insulin source, requiring replacement, while T2D has impaired signalling, requiring sensitisation or augmentation.

Marking notes. 1 mark — T1D: beta cell destroyed → no insulin; must replace exogenously. 1 mark — T2D: peripheral insulin resistance (receptor/signalling) + eventual beta-cell exhaustion; some insulin still present. 1 mark — T2D initial treatment targets resistance: metformin, lifestyle, GLUT4 upregulation by exercise. 1 mark — explains why treatments differ fundamentally (T1D = no source of insulin; T2D = impaired response to insulin that is still present).

Sample response (a). Long-term smokers accumulate mutations at an accelerating (non-linear) rate, reaching approximately 90 mutations by age 60. Non-smokers accumulate mutations at a slow, roughly linear rate, reaching approximately 17 by age 60. The difference in mutation burden at age 60 is approximately 73 mutations — roughly five-fold higher in smokers. The gap between the two groups widens with age, indicating that smoking has a compounding effect on mutation accumulation over time.

Sample response (b). Tobacco smoke contains polycyclic aromatic hydrocarbons (PAHs) — chemical carcinogens. PAHs are metabolised in lung cells to highly reactive epoxides that form covalent DNA adducts, primarily at guanine bases. These adducts cause guanine-to-thymine transversion mutations during DNA replication. If these mutations occur in key driver genes — particularly TP53 (removing the DNA-damage checkpoint) or oncogenes such as KRAS — they initiate or accelerate carcinogenesis. The rate is accelerating because early mutations in DNA repair genes (such as TP53) impair the cell's ability to correct subsequent carcinogen-induced errors, creating a compounding cycle: more mutations → less repair capacity → even faster mutation accumulation.

Sample response (c). The most likely source of mutations in non-smokers is random errors during DNA replication — polymerase misincorporation events that occur at a low but consistent rate every time a cell divides. These are biological errors intrinsic to cell division that accumulate throughout life regardless of environmental exposures. They cannot be eliminated by lifestyle choices because they are an inherent consequence of the chemistry of DNA replication, not of any external carcinogen. This is consistent with the data: the non-smoker line rises slowly and linearly with age, reflecting unavoidable replication errors accumulating over decades.

Marking notes. Part (a): 1 mark — correctly describes smoker rate as accelerating / non-linear and non-smoker rate as slow / roughly linear; 1 mark — estimates values at age 60 within ±15 mutations (smoker ~90, non-smoker ~17) and states the ~5-fold difference. Part (b): 1 mark — names PAHs as the carcinogen and describes DNA adduct formation at guanine; 1 mark — explains transversion mutation → driver gene mutation (TP53 / KRAS); 1 mark — explains compounding: early repair gene mutation → impaired correction of subsequent errors → accelerating rate. Part (c): 1 mark — identifies random DNA replication errors as the source; 1 mark — explains why lifestyle cannot eliminate them (inherent biochemical error rate in polymerase, not external).

Sample response. The claim is partially supported by evidence for modifiable risk-factor cancers and nutritional diseases, but is significantly limited by evidence from genetic diseases, biological carcinogen-driven cancers, and social determinants of health.

Support. Individual lifestyle choices substantially reduce risk for some non-infectious diseases. Tobacco smoking causes approximately 22% of Australian cancers via PAH-mediated DNA adducts that produce TP53 mutations — smoking cessation reduces lung cancer risk by ~85–90%. UV exposure drives melanoma through thymine dimer formation and BRAF V600E activation — sun protection and SPF use substantially reduce melanoma incidence. Dietary modification (reducing excess refined carbohydrates and saturated fat) and physical activity reduce Type 2 diabetes risk by over 50% in high-risk populations by improving insulin sensitivity and reducing hyperinsulinaemia. For these conditions, individual choice is a genuinely powerful preventive tool.

Genetic diseases — limit 1. Cystic fibrosis arises from two inherited non-functional CFTR alleles that produce absent or misfolded CFTR chloride channel protein, regardless of any lifestyle behaviour. No personal choice can alter the inheritance of these alleles or restore CFTR function. Similarly, Huntington's disease (autosomal dominant gain-of-function CAG expansion in huntingtin) and PKU (two non-functional PAH alleles) cannot be prevented by any individual behaviour. These diseases are not addressed by the lifestyle-based framing of the claim.

Biological carcinogens — limit 2. Cervical cancer caused by HPV-16 arises through E6-mediated p53 degradation and E7-mediated RB1 inactivation — a virological mechanism distinct from lifestyle-associated chemical carcinogens. Prevention requires the HPV vaccine (a medical intervention at the population level), not behavioural change. Most sexually active people are exposed to HPV regardless of behaviour, and the cancer mechanism operates through molecular pathways independent of diet or exercise.

Social determinants — limit 3. The framing of "individual choices" ignores that access to nutritious food, safe exercise environments, and preventive healthcare is shaped by income, education, geography, and occupational exposure — factors beyond individual control. Australian epidemiological data consistently show higher non-infectious disease rates in socioeconomically disadvantaged communities, independent of individual preferences.

Evaluative conclusion. Reducing the burden of non-infectious disease requires three levels of intervention, not just individual choice: (1) individual interventions for modifiable-risk diseases (smoking cessation, sun protection, dietary change); (2) medical/public health interventions for biologically caused diseases (HPV vaccination, iodine fortification, newborn genetic screening); and (3) systemic/structural interventions to address social determinants (food policy, urban planning, equitable healthcare access). The claim is partially valid for lifestyle-modifiable diseases but incorrect as a universal claim — genetic susceptibility, biological carcinogens, and social context all operate independently of individual choice.

Marking criteria.

• 1 mark — Identifies at least one disease category where individual choice is genuinely preventive; gives a named disease with mechanism (e.g. smoking → PAHs → TP53 mutation → lung cancer; UV → thymine dimers → BRAF V600E → melanoma).
• 1 mark — Explains a genetic disease category that lifestyle cannot prevent; names a specific disease with correct mechanism (CF/CFTR, Huntington's/huntingtin CAG, or PKU/PAH enzyme).
• 1 mark — Explains a biological carcinogen category where medical (not lifestyle) intervention is required; names HPV and at least one of the E6/p53 or E7/RB1 mechanisms.
• 1 mark — Addresses social determinants or structural factors that limit the effectiveness of individual choice as a prevention strategy.
• 1 mark — Correctly distinguishes among individual, medical/public health, and systemic interventions as the three required components of non-infectious disease reduction.
• 1 mark — Reaches an explicit, nuanced evaluative conclusion: the claim is partially valid but incorrect as a universal statement; frames the limitation clearly using biological evidence from at least three categories.