MRCS Part B Revision · Applied surgical science and critical care
Potassium — MRCS Part B Applied surgical science and critical care
By Dr Richard Miller, MBChB FRCS · Reviewed
Potassium 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 Potassium station
Potassium
Potassium is the major cation (positive ion) inside animal cells, and it is thus important in maintaining fluid and electrolyte balance in the body.
Balance
Aldosterone: a steroid hormone of the adrenal cortex. Stimulates absorption of sodium in the DCT of the kidney, and several other organs, at the expense of potassium loss through active exchange at the cell membrane
Acid-base balance:potassium and H+ are exchanged at the cell membrane, so that an increase of one ion leads to increased exchange with the other, e.g. acidosis leads to hyperkalaemia and vice versa. Such membrane exchange occurs in the kidney tubules as well as other cells
Renal exchange: increased flow leads to potassium loss – this is one way in which diuretics promote hypokalaemia
Insulin: stimulates potassium intake into cells, reducing the serum level
Images from this station
What are you asked at the Potassium station?
The station runs to 14 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- What are the causes of hyperkalaemia?
- What symptoms and signs might the patient have?
- What investigations would you request?
- An ECG is performed. What does it show?
- How would you treat hyperkalaemia?
- What is the distribution of potassium in the body?
- What is the normal range for serum potassium?
- How is potassium regulated?
- What are the causes of hypokalaemia?
- What are the signs, symptoms associated with hypokalaemia?
- What does the ECG show?
- How would you treat a post-operative low potassium of 2.8mmol/L?
And 2 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 Potassium station ask?
It opens with "What are the causes of hyperkalaemia?" and runs to 14 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