HSC Exam Practice

Sample response. GFR (glomerular filtration rate) is a measure of the rate at which the kidneys filter blood, expressed in mL/min/1.73 m². A GFR below 15 mL/min classifies a patient as having end-stage renal disease (ESRD).

Marking notes. 1 mark for defining GFR as the rate of blood filtration by the kidneys (accept equivalent); 1 mark for correctly stating the ESRD threshold as below 15 mL/min.

Sample response. The two most common causes of ESRD in Australia are (1) type 2 diabetes (~37% of cases) — chronic hyperglycaemia damages glomerular capillaries (diabetic nephropathy), thickening the basement membrane and causing glomerulosclerosis, progressively reducing filtration; and (2) hypertension (~25% of cases) — sustained high blood pressure damages glomerular membranes and walls of renal blood vessels, reducing functional nephron numbers over time.

Marking notes. 1 mark per cause identified by name; 1 mark per mechanism correctly described (max 2 mechanisms). Accept glomerulosclerosis / basement membrane thickening for diabetes; accept glomerular or vascular damage / sclerosis for hypertension.

Sample response. Urea is a small molecule (molecular mass ~60 Da) that is small enough to diffuse across the pores of the dialysis membrane; this movement is driven by its concentration gradient (high in blood, near-zero in dialysate). Plasma proteins (e.g. albumin, molecular mass ~69,000 Da) are far too large to pass through the membrane pores and are retained in the blood.

Marking notes. 1 mark for identifying that urea moves by diffusion down its concentration gradient and is small enough to cross the membrane; 1 mark for stating that plasma proteins are too large to cross the semi-permeable membrane (size/molecular exclusion). Do not accept "active transport" for urea movement.

Sample response. Acute rejection occurs days to weeks post-transplant; it is mediated by cytotoxic T cells (CD8+) recognising the donor kidney's foreign HLA antigens and attacking the graft; managed with high-dose corticosteroids and adjustment of immunosuppressant drugs. Chronic rejection develops over months to years; it involves slow immune-mediated fibrosis and scarring of the transplant (often antibody-driven in addition to T-cell involvement); managed by optimising ongoing immunosuppression — ultimately the graft may fail and the patient returns to dialysis or re-listing.

Marking notes. Acute rejection: 1 mark for timing (days–weeks); 1 mark for mechanism (T-cell mediated / immune attack on donor HLA antigens) and management (corticosteroids / adjust immunosuppressants). Chronic rejection: 1 mark for timing (months–years); 1 mark for mechanism (fibrosis / immune-mediated scarring) and management (optimise immunosuppression; eventual re-listing). Full marks require all four elements across the two types.

Sample response. If dialysate flowed in the same direction as blood (co-current), the dialysate would quickly equilibrate with the urea concentration in the blood and no further net diffusion would occur before the blood exited the dialyser. By flowing counter-current (opposite direction), fresh low-urea dialysate always encounters blood that still has a higher urea concentration at every point along the dialyser. This maintains a concentration gradient across the entire membrane length, maximising the total amount of urea removed per session.

Marking notes. 1 mark for explaining that co-current flow would lead to equilibration and loss of gradient; 1 mark for correctly describing that counter-current flow ensures the gradient is maintained along the full dialyser length; 1 mark for stating the consequence — maximum urea removal per session.

Sample response. Plasma urea oscillates repeatedly: it falls sharply during each haemodialysis session (dropping from approximately 28–32 mmol/L to approximately 9–11 mmol/L) then rises progressively between sessions. The pattern repeats six times over the two weeks, with urea never returning to the normal range (2.5–7.5 mmol/L) but remaining below the uraemia threshold for most of the period.

Marking notes. 1 mark for identifying the oscillating/sawtooth pattern (rises between sessions, drops during sessions); 1 mark for noting that urea does not return to normal between sessions (or that the post-HD level is still above normal).

Sample response. The plasma urea comes closest to the uraemia threshold (~35 mmol/L) before the day-8 dialysis session, where it reaches approximately 32 mmol/L. This is because the interval between the day-5 and day-8 sessions is 72 hours (3 days) — the longest gap in the schedule — compared with 48 hours for the other inter-session intervals. With no dialysis occurring over the weekend, urea continues to accumulate in the blood (from protein metabolism producing nitrogenous waste) for a longer period, resulting in a higher pre-session peak. A continuous-clearance method would prevent this accumulation.

Marking notes. 1 mark for identifying day 8 (or the pre-day-8 session) as the highest pre-session peak; 1 mark for identifying that the day 5–8 interval is the longest (72 h vs 48 h); 1 mark for explaining that urea accumulates continuously during the inter-session interval so a longer interval produces a higher peak.

Sample response. A functioning transplanted kidney filters blood continuously — 24 hours a day, 7 days a week — removing urea and other waste solutes as they are produced by protein metabolism, so urea concentrations remain within the normal range without oscillation. Haemodialysis is intermittent (3 sessions per week), so urea accumulates during the gaps between sessions and is only cleared during the sessions, producing the oscillating sawtooth pattern visible in the graph.

Marking notes. 1 mark for explaining that a transplanted kidney provides continuous clearance (24/7); 1 mark for explicitly contrasting this with the intermittent nature of haemodialysis as the cause of the oscillating pattern.

Error 1 — "uses the same artificial membrane as haemodialysis." Peritoneal dialysis does not use an artificial membrane at all. It uses the patient's own peritoneum (the abdominal lining) as the biological semi-permeable membrane across which waste solutes diffuse into the dialysate that is introduced into the peritoneal cavity. Haemodialysis uses a synthetic hollow-fibre membrane inside a dialyser machine. [1 mark for identifying the error; 1 mark for the correct biology.]

Error 2 — "patients on peritoneal dialysis never need to worry about infection risks unique to this method." Peritoneal dialysis carries a specific and significant infection risk: peritonitis — bacterial infection of the peritoneal cavity, typically via the permanent abdominal catheter insertion site. Peritonitis is a leading complication of peritoneal dialysis and can be life-threatening if untreated. [1 mark for identifying the error; 1 mark for correctly naming peritonitis as the specific risk.]

Marking notes. 2 marks per error (1 for identification, 1 for correct biology). Accept any other factual error in the extract if correctly justified (e.g. "equally effective" — PD provides more continuous but lower per-session clearance than HD; the modalities are not directly equivalent). Maximum 2 errors.

Sample response. The claim that kidney transplantation is always superior overstates the evidence; transplantation is the preferred option for most eligible patients but is not universally appropriate. A nuanced evaluation across several criteria reveals why. In terms of waste removal mechanism, haemodialysis removes urea and excess solutes by diffusion across a synthetic semi-permeable membrane three times per week; it is effective at reducing plasma urea per session but is inherently intermittent — waste accumulates between sessions, plasma K&sup+; can reach dangerous levels before the next appointment, and dietary restrictions are stringent. A functioning transplanted kidney performs continuous filtration 24 hours a day, removing waste as it is produced and maintaining plasma biochemistry within normal limits; it also restores endocrine functions (erythropoietin, activated vitamin D) that dialysis cannot replicate. Transplantation is therefore mechanistically superior for waste removal and homeostatic maintenance. However, transplantation carries a specific immunological challenge: the recipient's immune system recognises the donor kidney's HLA antigens as foreign, triggering T-cell mediated attack (acute rejection, days–weeks) or slow immune-mediated fibrosis (chronic rejection, months–years). Lifelong immunosuppressant therapy — for example tacrolimus and prednisolone — is required to prevent rejection; the trade-off is increased susceptibility to opportunistic infection and certain cancers (especially skin cancer and lymphoma) due to reduced immune surveillance. From a quality of life perspective, haemodialysis patients average 12 hours per week at a dialysis centre, experience post-session fatigue, and face severe dietary restrictions. Transplant recipients, after a recovery period, generally live near-normal lifestyles with fewer restrictions. ANZDATA data show substantially better five-year survival for transplant recipients (88–97%) compared to haemodialysis patients (48–72%) across adult age groups. The claim fails when patient-specific factors are considered. Transplantation is not appropriate for all patients: those with significant surgical risk (heart disease, severe obesity), older frail patients for whom surgical risk may exceed benefit, or patients unable to tolerate immunosuppression may be better served by dialysis long-term. Furthermore, organ shortage — approximately 2,700 Australians are on the kidney transplant waiting list with a median wait of 3.5 years — means dialysis is not just a choice but a necessity for many patients who cannot access a donor organ. In conclusion, transplantation is superior for most eligible patients, particularly younger patients in good health with an available donor, because it provides continuous filtration, restores hormonal functions, improves survival and quality of life. However, it is not universally superior because surgical risk, immunological trade-offs, comorbidities and organ availability mean dialysis remains the appropriate and sometimes the only treatment for a significant subset of patients. Treatment choice must be patient-specific, not modality-universal.

Marking notes. 1 mark — Compares mechanism: dialysis as intermittent diffusion across synthetic membrane vs transplant as continuous glomerular filtration; notes dialysis cannot replicate hormonal functions. 1 mark — Explains HLA rejection mechanism correctly and names at least one immunosuppressant drug. 1 mark — Addresses quality of life criterion with specific reference to dialysis burden (hours, fatigue, dietary restrictions) versus near-normal lifestyle post-transplant. 1 mark — References quantitative survival data (ANZDATA or equivalent) to support transplant's outcome advantage. 1 mark — Identifies patient-specific factors that make dialysis appropriate: surgical risk, comorbidities, age, frailty. 1 mark — Identifies organ shortage/waiting time as a systemic barrier that makes dialysis necessary for many patients regardless of preference. 1 mark — Reaches an explicit evaluative judgement that rejects "always superior" and correctly frames the decision as patient-specific and condition-dependent, using evidence from multiple criteria.