MRCS Part B Revision · Anatomy
Diaphragm — MRCS Part B Anatomy
By Dr Richard Miller, MBChB FRCS · Reviewed
Diaphragm is a surgical anatomy station. Three of the seventeen examined stations in the MRCS Part B OSCE are surgical anatomy. The anatomy stations put a prosected specimen, a model or an image in front of you and ask you to identify structures and say what they do, what they relate to and what happens when they are injured.
What you need to know for the Diaphragm station
The diaphragm from below with the structures that pass through it, and the three openings every candidate is asked to level.
Structure
A domed sheet of muscle inserting into a central tendon shaped like a trefoil, on which the heart sits. Its sternal part arises from the back of the xiphoid, its costal part from the inner surfaces of the lower six ribs and their cartilages, and its lumbar part from the two crura and the arcuate ligaments. The right crus, from L1–3, is longer and splits to enclose the oesophagus; the left crus arises from L1–2. The medial arcuate ligament bridges psoas and the lateral arcuate ligament bridges quadratus lumborum; the median arcuate ligament joins the two crura in front of the aorta. It is supplied by the phrenic nerves (C3, 4, 5), each also giving sensation to the central part, so irritation is felt at the shoulder tip.
The three main openings
- Caval opening, T8 — in the central tendon. Inferior vena cava and the right phrenic nerve. Being in the tendon, it widens on inspiration and aids venous return.
- Oesophageal hiatus, T10 — in the muscle of the right crus. Oesophagus, the anterior and posterior vagal trunks, and oesophageal branches of the left gastric vessels. The muscle sling helps prevent reflux.
- Aortic hiatus, T12 — behind the diaphragm, under the median arcuate ligament, so the aorta is not squeezed. Aorta, thoracic duct and the azygos vein.
“8, 10, 12; vena cava, oesophagus, aorta” — and each opening's letter count matches its level.
Other structures crossing it
The sympathetic trunks pass behind the medial arcuate ligaments; the greater, lesser and least splanchnic nerves pierce the crura; the hemiazygos vein passes through the left crus; the superior epigastric vessels pass between the sternal and costal parts (the foramen of Morgagni), and the subcostal nerves and vessels behind the lateral arcuate ligaments. The left phrenic nerve pierces the muscle of the left dome on its own.
Development
Four sources: the septum transversum makes the central tendon, the pleuroperitoneal membranes close the canals on each side, the dorsal mesentery of the oesophagus makes the crura, and the body wall contributes the peripheral muscle, which is why the phrenic nerves come from the cervical segments where the septum transversum began. The openings follow: the caval opening sits in the septum-derived tendon, the oesophageal hiatus in the mesentery-derived crus, and the aorta lies behind everything, between the crura.
Congenital diaphragmatic hernia
Bochdalek, posterolateral and usually left-sided, from failure of the pleuroperitoneal membrane, is the common one and presents at birth with respiratory distress from pulmonary hypoplasia. Morgagni, anterior and retrosternal, is rarer and often found later. An oesophageal hiatus hernia is acquired, not congenital.
Images from this station
What are you asked at the Diaphragm station?
The station runs to 13 questions over nine minutes. A prompt the station repeats — against each marker on a specimen, for instance — is listed once here; the model answers are in the question bank.
- What are the name openings in the diaphragm?
- At what vertebral level are these openings found
- What structures pass through the diaphragmatic main openings?
- How do the diaphragmatic openings correspond to the embryological development of the diaphragm?
- What other structures pass through the diaphragm?
- Identify structures 1-3
- Identify structures 4-6
- Identify vessels 7-8
- Identify structures 9-11
- Identify structures 12-14
- Identify vessel 15
- What are the common sites for congenital diaphragmatic hernia?
How is the anatomy station marked in MRCS Part B?
An anatomy station is marked out of 20, and all 20 marks go to clinical knowledge and its application. No communication, technical-skill or professionalism marks are available in an anatomy bay, which is unusual: in most other stations more than half the marks sit outside your factual knowledge.
FAQ
What does the Diaphragm station ask?
It opens with "What are the name openings in the diaphragm?" and runs to 13 questions over nine minutes. An anatomy station is marked out of 20, and all 20 marks go to clinical knowledge and its application. No communication, technical-skill or professionalism marks are available in an anatomy bay, which is unusual: in most other stations more than half the marks sit outside your factual knowledge.
How many anatomy stations are there in MRCS Part B?
Three of the seventeen examined stations are surgical anatomy, and two more are surgical pathology. Together they make up the five Anatomy and surgical pathology stations of the Applied Knowledge component.
How are the anatomy stations marked?
Each station is marked out of 20 and all 20 marks are awarded for clinical knowledge and its application. You also receive a separate global rating of pass, borderline or fail for the station.
Are the specimens real?
Yes. Anatomy bays use prosected cadaveric specimens, models and images. Candidates report being shown photographs of specimens as well as the specimens themselves, so practise on real dissection photography rather than diagrams alone.
What happens if I get an early question wrong?
The intercollegiate guidance says that if a wrong answer would affect your performance on later questions at the same station, the examiner gives you the correct answer before moving on. One mistake does not have to cost you the whole bay.
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 anatomy stations.
More anatomy stations