Thorax: MRCEM Primary questions with answers

Anatomy carries 60 of the 180 questions in MRCEM Primary, and Thorax is one of its topics. You study the bones, muscles and wall movements of the chest, then the mediastinum, heart and pericardium, lungs and pleurae, oesophagus, thymus and tracheobronchial tree, with the thoracic inlet and outlet and the dermatomes of the trunk. This underpins chest drain insertion and the assessment of chest trauma.

5 free questions below, each with the answer and the full explanation · 170 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · Structures of the Thorax-Heart and Pericardium

   A man aged 52 is unwell with crushing central chest pain. An ECG shows significant ST segment changes. Angiography shows occlusion of the circumflex branch of the left coronary artery.

   In which area of the heart does this condition most commonly result in ischemia?

   1. AAnterior part of the left ventricle
   2. BAnterior part of the right ventricle
   3. CInferior part of the right ventricle
   4. DPosterior part of the left ventricle
   5. EInferior part of the left ventricle
   Show answer and explanation

   Answer: D. The circumflex runs to the left border and posterior surface, supplying the left atrium and posterior left ventricle.

   • A: Anterior left ventricle is anterior interventricular territory.
   • B: Anterior right ventricle is supplied by right coronary and anterior interventricular branches.
   • C: Inferior right ventricle is right coronary territory.
   • E: Inferior wall is typically right coronary territory.

   Full explanation: Structures of the Thorax-Heart and Pericardium

   Heart and pericardium

    

   Pericardium

   Fibro-serous, fluid-filled sack that surrounds the muscular body of the heart and the roots of the great vessels. Great vessels include: The aorta, pulmonary artery, pulmonary veins, and the superior and inferior vena cavae. 

   Pericardium is made up of two main layers fibrous and serous pericardium. 

   Fibrous Pericardium: Surrounds the heart and forms a protective covering for the cardiac Chambers. It also acts as an anchor for the heart that keeps it restricted in the chest cavity by binding itself to the adjoining regions. It also prevents the overexpansion of the heart when there is a rise in the volume of blood inside the body. 

   Serous Pericardium: It is closed sac within fibrous pericardium having Visceral & Parietal layer, separated by the fluid-filled pericardial cavity 

   • The visceral layer of serous pericardium (epicardium): covers the surface of the heart. It also reflects onto the great vessels This transparent membrane surrounds and protects the heart. It also contains a thin film of serous fluid that allows the heart to move and provides it with a frictionless environment to expand as well as contract. 
   • The parietal layer lines the internal surface of the fibrous pericardium. 

    

   Innervation of the pericardium

   The visceral pericardium: Receives autonomic innervation from the cardiac plexus. 

   The parietal and fibrous pericardium are innervated mainly by the phrenic nerve, for this reason that pain related to a pericardial problem may be referred to the supraclavicular region of the shoulder or the lateral neck area. 

    

   Clinical pearls:

     Pericardiocentesis:  surgical puncture of the pericardial cavity for the aspiration of fluid, which is necessary to relieve the pressure of accumulated fluid in the heart. A needle is inserted into the pericardial cavity 1 – 2 cm inferior to the left of the xiphochondral junction (between the xiphisternum and the left costal margin)  

   Complications: 

   • Pneumothorax secondary to lung puncture 
   • Aspiration of ventricular blood instead of pericardial blood 
   • Laceration of ventricular epicardium or myocardium 
   • Laceration of coronary artery or vein 
   • Ventricular fibrillation 
   • Puncture of great vessels with worsening pericardial tamponade 

    

   Heart:

   The heart is a myocardial muscular pump consisting of four chambers, two auricles, four valves, and a muscular septum all enclosed within a fluid filled sac, the pericardium 

   The heart wall is composed of three distinct layers:  

   1: The epicardium (outermost layer) 

   2: The myocardium (middle layer, comprised of muscular fibers) 

   3: The endocardium (innermost layer), lines the cavities and valves of the heart 

    

   Borders

   The heart has four borders: 

   • The right border: right atrium and is in line with the superior and inferior vena cavae. 
   • The left border: the left ventricle and very slightly by the left auricle.
   • The superior border: is where the great vessels enter and leave the heart. It is formed by both atria 
   • The inferior border: The right ventricle, and slightly by the left ventricle near the apex. 

    

   The heart has been described as “a pyramid which has fallen over”. The apex of this pyramid pointing in an anterior-inferior direction. 

   In its typical anatomical orientation, the heart has 5 surfaces, formed by different internal divisions of the heart: 

   • Anterior (or sternocostal): Right ventricle. 
   • Posterior (or base): Left atrium. 
   • Inferior (or diaphragmatic): Left and right ventricles. 
   • Right pulmonary: Right atrium. 
   • Left pulmonary: Left ventricle. 

   NOTE: Esophagus lies immediately posterior to the base (left atrium). The term cardiac dysphagia referred as impairment of swallowing due to esophageal compression from a dilated left atrium 

    

   Cardiac Conduction system: collection of specialized conduction cells and nodes that initiate and co-ordinate contraction of the heart muscle. It consists of: 

   • Sinoatrial node 
   • Atrioventricular node 
   • Atrioventricular bundle (bundle of His) 
   • Purkinje fibres 

   The sinoatrial node, the cardiac pacemaker, is located at the superior end of the crista terminalis at the junction of the superior vena cava and the right atrium. Impulses generated by the sinoatrial node are propagated by the atrial myocardium to the atrioventricular node, Here the impulse is briefly delayed before being spread to the ventricles. 

   The atrioventricular bundle (bundle of His) is a direct continuation of the atrioventricular node and continues along the lower margin of the membranous part of the interventricular septum before dividing into the right and left bundles which continue themselves as Purkinje fibres, the final component of the conducting system. 

    

   High yield:

   • The sinoatrial node (SAN) is the primary pacemaker. The SAN will normally discharge at a rate of 60-100 bpm. if the impulse generated in the Sa node is blocked, a group of cells further down the heart will become its pacemaker. 
   • The atrioventricular node (AVN) is the secondary pacemaker and will normally discharge at about 40-60 beats per minute. AV nodal abnormality is responsible for prolonged PR interval (1st degree heart block)  

   Great Vessels of the heart: Mnemonic (SIPPA)

   •  Superior vena cava 
   • Inferior vena cava 
   • Pulmonary arteries 
   • Pulmonary veins 
   • Aorta 

   1. Superior vena cava: Receives deoxygenated blood from the upper body excluding the lungs and heart superior to the diaphragm, delivering it to the right atrium 
   2. Inferior vena cava: receives deoxygenated blood from the lower body inferior to the diaphragm, delivering it back to the heart 

   3. Pulmonary arteries: Receive deoxygenated blood from the right ventricle and deliver it to the lungs for gas exchange to take place through pulmonary valve 

   4. Pulmonary veins: Receive oxygenated blood from the lungs, delivering it to the left side of the heart to be pumped back around the body. 

   5. Aorta: Largest artery carry oxygenated blood pumped by left side of the heart to all body. The aorta arises from the aortic orifice at the base of the left ventricle, with inflow via the aortic valve. First segment is known as the ascending aorta, which lies within the pericardium. The second continuous segment is the arch of the aorta, from which branch the major arteries to the head, neck and upper limbs. These are: 

   • Brachiocephalic trunk 
   • Left common carotid artery  
   • Left subclavian artery  

   After the arch of the aorta, the aorta then becomes the descending aorta which continues down through the diaphragm into the abdomen 

    

   High yield:

   Areas of auscultation of heart: 

   Cardiac blood supply:

   1: The Right coronary artery supplies:

   • The right atrium and most of right ventricle 
   • The sinoatrial and atrioventricular node 
   • Posterior interventricular artery, also known as the posterior descending artery (PDA), is a branch of the right coronary artery supply posterior 1/3 of interventricular septum and posterior aspect of right and left ventricle. Most common artery infracted in posterior wall MI

   Note: Occlusion of right coronary artery can result into ischemia of SA and AV node and subsequently bradycardia or heart block  

   2: The left coronary artery supplies via main branches below:

   •  Anterior interventricular branch (LAD)
   • LCX

   High yield:

   Localization of lesion in ST elevation MI and occluded coronary artery. Only remember myocardial area and occluded coronary artery.

   Easy to remember:

   • Septal and Anterior wall MI: LAD (Anterior interventricular branch) occluded.
   • Lateral wall MI: LCX occluded.

2. Question 2 · Structures of the Thorax-Lungs and Pleurae

   A patient aged 62 with pericardial effusion comes to a local hospital for aspiration of pericardial fluid by pericardiocentesis. The needle is inserted into the pericardial cavity through which of the following intercostal spaces adjacent to the sternum?

   1. ARight fourth intercostal space
   2. BRight fifth intercostal space
   3. CRight sixth intercostal space
   4. DLeft fifth intercostal space
   5. ELeft fourth intercostal space
   Show answer and explanation

   Answer: D. Pericardiocentesis needle entry is in the fifth intercostal space, 1-2 cm lateral to the apex beat, over the rib's upper border.

   • A: Not the recommended site; fifth space is used.
   • B: Right-sided approach is away from the apex beat.
   • C: Not the recommended site; fifth space is used.
   • E: Fourth space is not the recommended site.

   Full explanation: Structures of the Thorax-Lungs and Pleurae

   LUNG:

   Lungs lie either side of the mediastinum, within the thoracic cavity. Each lung is surrounded by pleural cavity, which is formed by the visceral and parietal pleura. Lungs are suspended from the mediastinum by the lung root a collection of structures entering and leaving the lungs. 

   Each root contains a bronchus, pulmonary artery, two pulmonary veins, bronchial vessels, pulmonary plexus of nerves and lymphatic vessels. All these structures enter or leave the lung via the hilum. The hila are located at the level of the 3rd – 4th costal cartilage anteriorly, and the T5 – T7 vertebrae posteriorly. In the mediastinum, the vagus nerves pass immediately posteriorly to the lung roots, while the phrenic nerves pass immediately anterior to them. 

   Note: The right lung is normally a little larger than the left lung because the middle mediastinum, containing the heart, bulges more to the left than to the right. 

    

   Relations:

   Medial surface of the right lung lies adjacent to	Medial surface of the left lung lies adjacent to
   The heart	The heart
   The inferior vena cava	The aortic arch
   The superior vena cava	The thoracic aorta
   The azygos vein.	The oesophagus
   The oesophagus

   Lymphatic vessels of the lung: Drain the bronchial tree, pulmonary vessels, and connective tissue septa. Run along bronchioles and bronchi toward the hilum, where they drain to the pulmonary (intrapulmonary) and then bronchopulmonary nodes, which in turn drain to the inferior and superior tracheobronchial nodes, the tracheal (paratracheal) nodes, broncho mediastinal nodes and trunks, and eventually to the thoracic duct on the left and right lymphatic duct on the right. Are not present in the walls of the pulmonary alveoli. 

    

   1. Thoracic duct: Begins in the abdomen at the cisterna chyli, which is the dilated junction of the intestinal, lumbar, and descending intercostal trunks. Drains the lower limbs, pelvis, abdomen, left thorax, left upper limb, and left side of the head and neck. Passes through the aortic opening of the diaphragm and ascends through the posterior mediastinum between the aorta and the azygos vein, Arches laterally over the apex of the left pleura and between the left carotid sheath in front and the vertebral artery behind, runs behind the left internal jugular vein, and then usually empties into the junction of the left internal jugular and subclavian veins. 

     

   1. Right lymphatic duct: Drains the right sides of the thorax, upper limb, head, and neck. Much of the left lower lung lobe drains to the right lymphatic duct. Empties into the junction of the right internal jugular and subclavian veins.

    

   Vasculature of lung:

   • The pulmonary artery conveys the venous blood to the lungs. 
   • The pulmonary veins commence in the pulmonary capillaries, open into the left atrium of the heart, conveying oxygenated blood to be distributed to all parts of the body by the aorta. 

   • The bronchial arteries supply blood for the nutrition of the lung. 
   • The bronchial vein is formed at the root of the lung, receiving superficial and deep veins corresponding to branches of the bronchial artery 

   Innervation of lung: The nerve supply to the lungs is from the anterior and posterior pulmonary plexus (networks of nerves derived from the sympathetic trunk and vagus nerve). 

    

   Lobes of the lung: The left lung is divided into two lobes, an upper and a lower, by an interlobular fissure, which extends from the costal to the mediastinal surface of the lung both above and below the hilus.The superior lobe lies above and in front of this fissure, and includes the apex, the anterior border, and a considerable part of the costal surface and the greater part of the mediastinal surface of the lung. 

   The inferior lobe, the larger of the two, is situated below and behind the fissure, and comprises almost the whole of the base, a large portion of the costal surface, and the greater part of the posterior border. 

    The right lung is divided into three lobes, superior, middle, and inferior, by two interlobular fissures. the oblique fissure separates the inferior lobe from the superior and middle lobe whilst the horizontal fissure separates the superior lobe from the middle lobe. 

   Clinical pearls:

   Lung auscultation: to make it summarize remember, sounds heard anteriorly think upper lobes, sounds heard posteriorly think lower lobes and middle lobe will be heard to the right, near the midaxillary line 

   • The superior lobe of the right lung is best auscultated above the 4th rib anteriorly. 
   • The middle lobe of the right lung is best auscultated between the 4th and the 6th rib anteriorly. 
   • The superior lobe of the left lung is best auscultated above the 6th rib anteriorly. 
   • The inferior lobes of the left and right lungs are best auscultated in the back between T3 & T10. 

    

   Pancoast tumor (superior pulmonary sulcus tumor) is a malignant neoplasm of the lung aper may cause a lower trunk brachial plexopathy and a lesion of cervical sympathetic chain ganglion and Horner syndrome (ptosis, enophthalmos, miosis, anhidrosis, and vasodilation). 

   Thoracotomy: surgical procedure to gain access into the pleural space of the chest 

   •  Antero lateral thoracotomy muscle divided serratus anterior and pectoralis major 
   •  Antero lateral thoracotomy: Nerve injury long thoracic nerve  
   •  Post: lateral thoracotomy muscle divided latissimus dorsi 
   •  Post: lateral thoracotomy nerve injury: Thoracodorsal nerve injury 
   •  Internal mammary artery and Aorta are likely to be damaged also 

    

   Pleurae:

   Each lung is surrounded by a pleura. The pleura consists of two parts. 

   • The parietal pleura covers internal surface of the thoracic cavity. Parietal pleura includes mediastinal pleura, costal pleura and diaphragmatic pleura 
   •  The visceral pleura lines the lungs. 
   •  The two parts of the pleura are continuous with each other at the hilum of each lung. The potential space between these two parts of the pleura is called the pleural cavity. 

    

   Pleural Recesses: Pleural cavity is not completely filled by the lungs, anteriorly and posteroinferiorly, this gives rise to recesses where the opposing surfaces of the parietal pleura touch. 

   There are two recesses present in each pleural cavity, recesses are of clinical importance, as they provide a location where fluid can collect i.e., in a pleural effusion 

   • Costo diaphragmatic: located between the costal pleurae and the diaphragmatic pleura. 
   • Costo mediastinal: located between the costal pleurae and the mediastinal pleurae, behind the sternum. 

   Surface markings of pleura:

   • Superiorly the pleural cavity projects as much as 3-4 cm above the first costal cartilage but does not extend above the neck of rib 1. 
   • Inferiorly the pleura reflects onto the diaphragm above the costal margin. The inferior margin of the pleural cavity is a line that runs between rib 8 in the midclavicular line, rib 10 in the midaxillary line and T12 vertebra posteriorly. 

    

   Innervation of pleura

   • The parietal pleura is innervated by somatic afferent fibres. Parietal pleura includes mediastinal pleura, costal pleura and diaphragmatic pleura. 
   •  The costal pleura (pleura related to the ribs and intercostal spaces) is innervated by branches of the intercostal nerves, and pain is felt in relation to the thoracic wall.  
   • The diaphragmatic pleura and the mediastinal pleura are innervated mainly by the phrenic nerves (C3 – C5), and pain is referred the C3 – C5 dermatomes include lateral neck and supraclavicular region of shoulder. 
   • The visceral pleura is innervated by visceral afferent fibres, not sensitive to pain or temperature. 

    

   Clinical pearls:

    Thoracentesis (or pleural tap): is surgical entry through the thoracic wall into the pleural cavity for aspiration of fluid. An accumulation of fluid in the pleural cavity is known by clinical names, such as a hydrothorax (water), a hemothorax (blood), a chylothorax (lymph), and a pyothorax (pus).  

   Site: Pleural tap is performed at or posterior to the midaxillary line, one or two intercostal spaces below the fluid level ideal site is the sixth, seventh, eighth, or ninth intercostal space but not below the ninth intercostal space 

   Note: A needle should be inserted immediately above the superior margin of a rib to avoid injury to the intercostal neurovascular bundle, which travels in a notch below each rib 

    

    

    

    

    

3. Question 3 · Structures of the Thorax- Tracheobronchial Tree

   A patient aged 55 presents to the emergency department with a persistent cough and hemoptysis. On x-ray you note hilar lymphadenopathy. Which of the following lymph nodes are most likely enlarged?

   1. ACervical lymph node
   2. BSuperior tracheobronchial lymph node
   3. CAnterior mediastinal lymph node
   4. DInferior tracheobronchial lymph node
   5. EBronchopulmonary lymph node
   Show answer and explanation

   Answer: E. Bronchopulmonary nodes are most frequently involved in lung pathology, giving hilar lymphadenopathy.

   • A: Cervical nodes are not hilar.
   • B: Lies downstream of the bronchopulmonary nodes, not at the hilum.
   • C: Anterior mediastinal nodes are not hilar.
   • D: Carinal nodes lie downstream of the hilar nodes.

   Full explanation

   • The Bronchopulmonary lymph node most frequently involved in pathologies of lung.

4. Question 4 · Structures of the Thorax-Thymus

   During thymus surgery, which nerve is most commonly at risk of injury, potentially resulting in respiratory complications?

   1. AAccessory nerve
   2. BVagus nerve
   3. CHypoglossal nerve
   4. DPhrenic nerve
   5. ERecurrent laryngeal nerve
   Show answer and explanation

   Answer: D. The phrenic nerves are closely invested with the thymus laterally and can be injured, causing diaphragmatic dysfunction.

   • A: Lies in the neck, away from the thymus.
   • B: Lies lateral to the thymus region but not closely invested with it.
   • C: Lies in the neck, away from the thymus.
   • E: Lies in the tracheo-oesophageal groove, not closely invested with the thymus.

   Full explanation: Structures of the Thorax-Thymus

   The thymus

   lobulated lymphoid organ, located in the thoracic cavity and neck. In the adolescent, it is involved the development of the immune system 

   • Is a lobulated specialized primary lymphoid organ of the immune system, within which T-lymphocytes mature in the adolescent 
   • After puberty, it decreases in size and is slowly replaced by fat.  
   • It is mainly located within the superior mediastinum, posterior to the manubrium sterni.   
   • A certain degree of anatomical variation exists, and in some individuals, it can extend superiorly into the neck or inferiorly into the anterior mediastinum.  

   The thymus has an asymmetrical, flat shape with a lobular structure.  

   • The lobules are comprised of a series of follicles, which consist of a medullary and a cortical portion.  
   • The cortex: Is located peripherally within each follicle and is mainly composed of lymphocytes, supported by a network of finely branched epithelial reticular cells.  
   • The medulla: Is located centrally within each follicle and lymphoid cells are fewer than cortical portion. There are also a number of concentric arrangements of epithelial cells known as Hassall’s corpuscles. 

   Vasculature of the thymus:

   The arterial supply to the thymus gland is via the anterior intercostal arteries and small branches from the internal thoracic arteries. Venous blood drains into the left brachiocephalic and internal thoracic veins

   Innervation of the thymus:

   • The thymus gland is innervated by sympathetic nerves arising from the cervical sympathetic ganglia. These sympathetic nerves regulate blood flow to the thymus and play a role in its function. In addition, parasympathetic fibers from the vagus nerve may also contribute to the innervation of the thymus gland nevertheless an autonomic nervous system of both sympathetic and parasympathetic components plays a role in regulating function of thymus gland.

    

5. Question 5 · Thorax

   During a full neurological examination, what is the sensory level of the xiphisternum:

   1. AT3
   2. BT4
   3. CT5
   4. DT6
   5. ET7
   Show answer and explanation

   Answer: D. T6 corresponds to the level of the xiphisternum.

   • A: T3 is the third intercostal space.
   • B: T4 is the nipple level.
   • C: T5 lies midway between the nipples and xiphisternum.
   • E: T7 lies below the xiphisternum.

   Full explanation: Dermatomal Supply of Trunk

   Table: Dermatomal Supply of the Trunk

   Dermatome	Landmark
   T1	Medial antecubital fossa
   T2	Apex of axilla
   T3	Third intercostal space
   T4	Level of nipples (fourth intercostal space)
   T5	Fifth intercostal space
   T6	Level of xiphisternum
   T7	One-quarter distance between xiphisternum and umbilicus
   T8	One-half distance between xiphisternum and umbilicus
   T9	Three-quarter distance between xiphisternum and umbilicus
   T10	Level of umbilicus
   T11	Midway between umbilicus and inguinal ligament
   T12	Midpoint of inguinal ligament

   Dermatome Map. (Image by Grant, John Charles Boileau (An atlas of anatomy, / by regions 1962) [Public domain], via Wikimedia Commons)

   The T3 dermatome is best tested at the midclavicular line and the third intercostal space.

   The T4 dermatome is best tested at the midclavicular line and the fourth intercostal space, located at the level of the nipples.

   The T5 dermatome is best tested at the midclavicular line and the fifth intercostal space, located midway between the level of the nipples and the level of the xiphisternum.

   The T6 dermatome is best tested at the midclavicular line, located at the level of the xiphisternum.

   The T7 dermatome is best tested at the midclavicular line, one quarter the distance between the level of the xiphisternum and the level of the umbilicus.

   The T8 dermatome is best tested at the midclavicular line, one half the distance between the level of the xiphisternum and the level of the umbilicus.

   The T9 dermatome is best tested at the midclavicular line, three quarters of the distance between the level of the xiphisternum and the level of the umbilicus.

   The T10 dermatome is best tested at the midclavicular line, located at the level of the umbilicus.

   The T11 dermatome is best tested at the midclavicular line, midway between the level of the umbilicus and the inguinal ligament.

   The T12 dermatome is best tested at the midclavicular line, over the midpoint of the inguinal ligament.