MRCS Part B Revision · Applied surgical science and critical care
Calcium — MRCS Part B Applied surgical science and critical care
By Dr Richard Miller, MBChB FRCS · Reviewed
Calcium 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 Calcium station
Calcium
The plasma ionized calcium concentration is regulated to within very narrow limits (1.3 – 1.5 mmol/L). This is achieved by both the parafollicular cells of the thyroid gland, and the parathyroid glands constantly sensing the concentration of calcium ions in the blood flowing through them.
When the concentration rises the parafollicular cells of the thyroid gland increase their secretion of calcitonin (a proteinaceous hormone) into the blood.
At the same time the parathyroid glands reduce their rate of parathyroid hormone (or PTH, also a proteinaceous hormone) secretion into the blood. The resulting high levels of calcitonin in the blood stimulate the skeleton to remove calcium from the blood plasma, and deposit it as bone. The reduced levels of PTH inhibit removal of calcium from the skeleton.
The low levels of PTH have several other effects: they increase the loss of calcium in the urine, but more importantly inhibit the loss of phosphate ions via that route.
Phosphate ions will therefore be retained in the plasma where they form insoluble salts with calcium ions, thereby removing them from the ionized calcium pool in the blood. The low levels of PTH also inhibit the formation of calcitrol (1,25 dihydroxyvitamin D3) from cholecalciferol (vitamin D3) by the kidneys.
The reduction in the blood calcitrol concentration acts (comparatively slowly) on the epithelial cells (enterocytes) of the duodenum inhibiting their ability to absorb calcium from the intestinal contents.
When the plasma ionized calcium level is low or falls the opposite happens. Calcitonin secretion is inhibited and PTH secretion is stimulated, resulting in calcium being removed from bone to rapidly correct the plasma calcium level. The high plasma PTH levels inhibit calcium loss via the urine while stimulating the excretion of phosphate ions via that route.
They also stimulate the kidneys to manufacture calcitrol (a steroid hormone), which enhances the ability of the cells lining the gut to absorb calcium from the intestinal contents into the blood, by stimulating the production of calbindin in these cells. This is, however, a relatively slow process.
Thus fast short term regulation of the plasma ionized calcium levels primarily involves rapid movements of calcium into or out of the skeleton. Longer term regulation is achieved by regulating the amount of calcium absorbed from the gut or lost via the faeces.
What are you asked at the Calcium station?
The station runs to 15 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- How does the body store calcium?
- What is the normal serum level of calcium?
- Why must a calculation be performed to determine the 'corrected' calcium level?
- What are the causes of hypercalcaemia?
- What are the symptoms of hypercalcaemia?
- Describe the hormones responsible for regulating calcium levels
- What tests might be useful for investigating hypercalcaemia?
- What are the causes of hypocalcaemia?
- What are the signs and symptoms of hypocalcaemia?
- What do you understand by hyperparathyroidism?
- What are the causes of hyperparathyroidism?
- What are the causes of hypoparathyroidism?
And 3 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 Calcium station ask?
It opens with "How does the body store calcium?" and runs to 15 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