MRCS Part B Revision · Applied surgical science and critical care
AKI — MRCS Part B Applied surgical science and critical care
By Dr Richard Miller, MBChB FRCS · Reviewed
AKI is an applied surgical science and critical care station. Three of the seventeen examined stations in the MRCS Part B OSCE fall in this area. These stations ask you to interpret data and manage a sick surgical patient: a chart, a blood gas, an imaging study or a deteriorating patient on the ward, and the physiology underneath the decision.
What you need to know for the AKI station
Acute Kidney Injury
AKI can be caused by systemic disease (such as a manifestation of an autoimmune disease, e.g. lupus nephritis), crush injury, contrast agents, some antibiotics, and more. It is often multifactorial. The most common cause is dehydration and sepsis combined with nephrotoxic drugs, especially following surgery or contrast agents.
The causes of acute kidney injury are commonly categorized into prerenal, renal/intrinsic, and postrenal.
Pre-Renal
Prerenal causes of AKI are those that decrease effective blood flow to the kidney. These include systemic causes, such as low blood volume, low blood pressure, heart failure, liver cirrhosis and local changes to the blood vessels supplying the kidney. The latter include renal artery stenosis, or the narrowing of the renal artery which supplies the kidney with blood, and renal vein thrombosis, which is the formation of a blood clot in the renal vein that drains blood from the kidney.
Renal/Intrinsic
Sources of damage to the kidney itself are dubbed intrinsic. Renal/Intrinsic AKI can be due to damage to the glomeruli, renal tubules, or interstitium.
Common causes of each are glomerulonephritis, acute tubular necrosis (ATN), and acute interstitial nephritis (AIN), respectively. Other causes of intrinsic AKI are rhabdomyolysis and tumour lysis syndrome.
Post-Renal
Postrenal AKI is a consequence of urinary tract obstruction. This may be related to benign prostatic hyperplasia, kidney stones, obstructed urinary catheter, bladder stone, bladder, ureteral or renal malignancy. It is useful to perform a bladder scan or a post void residual to rule out urinary retention. In post void residual, a catheter is inserted immediately after urinating to measure fluid still in the bladder. 50-100 ml suggests neurogenic bladder dysfunction.
A renal ultrasound will demonstrate hydronephrosis if present. A CT scan of the abdomen will also demonstrate bladder distension or hydronephrosis. However, in AKI, the use of IV contrast is contraindicated as the contrast agent used is nephrotoxic.
Prognosis
Mortality remains high. Overall it is 20%; 30%, if the patient is referred to nephrology, 50% if dialysed, and 70% if on ITU
Depending on the cause, a proportion of patients (5-10%) will never regain full renal function, thus entering end-stage kidney failure and requiring lifelong dialysis or a kidney transplant. Patients with AKI are more likely to die prematurely after being discharged from hospital, even if their kidney function has recovered.
The risk of developing chronic kidney disease is increased (8.8-fold).
What are you asked at the AKI station?
The station runs to 17 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- PaO2 10 kPa (75 mmHg) (>10 kPa (75 mmHg) on air ) pH 7.17 (7.35 – 7.45) PaCO2 4.5 kPa (34 mmHg) (4.7 – 6.0 kPa (35 – 45 mmHg)) HCO3- 12 mmol l-1 (22 – 26 mmol l-1) BE - 15 mmol l-1 (+/- 2 mmol l-1) Applied Sciences: AKI
- What do the blood gas results demonstrate?
- Define the term pH
- Which organ systems are involved in regulating acid-base balance?
- How does the kidney absorb bicarbonate?
- Define the base deficit
- What is the definition of acute kidney injury?
- Name the causes of acute kidney injury
- What is acute tubular necrosis?
- Which common hospital-used medications can cause AKI?
- Describe the diagnostic criteria for acute kidney injury
- What is the RIFLE criteria?
And 5 more at this station.
How is the applied surgical science and critical care station marked in MRCS Part B?
Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
FAQ
What does the AKI station ask?
It opens with "PaO2 10 kPa (75 mmHg) (>10 kPa (75 mmHg) on air ) pH 7.17 (7.35 – 7.45) PaCO2 4.5 kPa (34 mmHg) (4.7 – 6.0 kPa (35 – 45 mmHg)) HCO3- 12 mmol l-1 (22 – 26 mmol l-1) BE - 15 mmol l-1 (+/- 2 mmol l-1) Applied Sciences: AKI" and runs to 17 questions over nine minutes. Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
What counts as applied surgical science in MRCS Part B?
Three of the seventeen examined stations: critical care management, interpretation of clinical data such as blood results and charts, and interpretation of visual information such as imaging and traces.
How is an applied science station marked?
Out of 20, with 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. Unlike the communication stations, none of the marks are for how you say it.
How much physiology do I need?
Enough to explain the decision you are making. The station rewards applying physiology to the patient in front of you rather than reciting it, so practise talking through why a number changes your management.
How many stations are in the MRCS Part B OSCE?
Seventeen examined stations of nine minutes each, with a minute to read the task before each one. Two preparation stations and at least one rest station bring the circuit to about twenty, and the exam takes about three and a half hours.
What is the pass mark for MRCS Part B?
There is no published pass mark. The cut score is set separately for Applied Knowledge and Applied Skills, for each circuit, by borderline regression. Published pass rates across the 2024/25 diets ranged from 51% to 66%.
Can I fail a station and still pass?
Yes. There is no rule about how many stations you may fail: the cut score applies to your total mark in each component, so a weak station costs the marks you lost on it and strong stations elsewhere can make them back. Applied Knowledge and Applied Skills are passed separately and must both be passed at the same sitting, so a strong anatomy performance cannot rescue a weak communication one.
Dr Richard Miller, MBChB FRCS
Station summaries are reviewed against the current intercollegiate MRCS syllabus and the published marking blueprint. Guidance changes between diets: check the royal colleges' own pages before relying on a date, a fee or a threshold.
Practise this station
The question bank carries the model answer to every question above, with the rest of the applied surgical science and critical care stations.
More applied surgical science and critical care stations