Thorax — MCQs
On this page
What is true about the coronary sinus?
Which of the following statements about the SA node is incorrect?
At which vertebral level does the oesophagus cross the diaphragm?
Which structure passes through the opening marked 3 in the diaphragm?
All of the following are true about coronary arteries, EXCEPT?
The third constriction of the esophagus is at the level of which anatomical landmark?
A 30-year-old man is diagnosed with a blockage of arterial flow in the proximal part of the thoracic aorta. Brachial arterial pressure is markedly increased, femoral pressure is decreased, and the femoral pulses are delayed. The patient shows no external signs of inflammation. Which of the following structures failed to develop normally?
The triangle of Koch is bound by all of the following except?
In a standing man, at the midaxillary line, where does the lower border of the pleura reach?
Which of the following statements about the esophageal hiatus is TRUE?
Thorax Indian Medical PG Practice Questions and MCQs
Question 261: What is true about the coronary sinus?
- A. It is the remnant of the left horn of the sinus venosus.
- B. The great, middle, and small cardiac veins drain into it.
- C. The Thebesian valve guards its opening.
- D. All of the above. (Correct Answer)
Explanation: The **coronary sinus** is the primary venous channel of the heart, located in the posterior part of the atrioventricular groove. It is approximately 2-3 cm long and drains most of the venous blood from the myocardium into the right atrium. ### **Detailed Breakdown:** * **Option A (Embryology):** During heart development, the venous end of the heart tube is the sinus venosus, which has two horns. While the right horn is incorporated into the right atrium (forming the *sinus venarum*), the **left horn** undergoes regression and persists as the **coronary sinus** and the oblique vein of the left atrium (of Marshall). * **Option B (Tributaries):** The coronary sinus receives blood from several major cardiac veins [1]: * **Great cardiac vein:** Accompanies the LAD artery [1]. * **Middle cardiac vein:** Accompanies the posterior interventricular artery. * **Small cardiac vein:** Accompanies the right marginal artery. * *Note:* The anterior cardiac veins are an exception; they drain directly into the right atrium. * **Option C (Anatomy):** The coronary sinus opens into the right atrium between the opening of the IVC and the tricuspid orifice. This opening is guarded by a semicircular endocardial fold known as the **Thebesian valve** (Valve of the coronary sinus). ### **High-Yield NEET-PG Pearls:** * **Location:** It lies in the posterior coronary sulcus. * **Thebesian Veins:** These are the smallest cardiac veins (*venae cordis minimae*) that drain directly into the heart chambers, bypassing the coronary sinus. * **Clinical Significance:** The coronary sinus is used as a landmark for **electrophysiological studies** and is the site for lead placement in **cardiac resynchronization therapy (CRT)**. It is also a site for retrograde cardioplegia delivery during cardiac surgery [1]. * **Valve of IVC:** Do not confuse the Thebesian valve with the **Eustachian valve**, which guards the opening of the Inferior Vena Cava.
Question 262: Which of the following statements about the SA node is incorrect?
- A. It is supplied by the nodal artery.
- B. It functions as the primary pacemaker of the heart.
- C. It is primarily supplied by the left vagus nerve. (Correct Answer)
- D. It is composed of nodal cells and connective tissue.
Explanation: The **Sinoatrial (SA) node** is the primary pacemaker of the heart, located in the upper part of the sulcus terminalis near the opening of the superior vena cava [1]. ### **Why Option C is Incorrect (The Correct Answer)** The SA node is primarily supplied by the **right vagus nerve**, while the Atrioventricular (AV) node is primarily supplied by the **left vagus nerve**. Parasympathetic stimulation via the right vagus slows the rate of impulse formation at the SA node (negative chronotropy) [2]. In contrast, the left vagus primarily influences conduction velocity through the AV node. ### **Analysis of Other Options** * **Option A:** The SA node is supplied by the **nodal artery**, a branch of the coronary arteries. In approximately 60% of individuals, it arises from the Right Coronary Artery (RCA), and in 40%, it arises from the Left Circumflex Artery (LCX). * **Option B:** It is the **primary pacemaker** because it possesses the highest rate of spontaneous depolarization (60–100 bpm), overriding other latent pacemakers [3]. * **Option C:** Histologically, it consists of specialized **P-cells (pacemaker cells)**, transitional cells, and a dense matrix of **connective tissue** collagen. ### **High-Yield Clinical Pearls for NEET-PG** * **Location:** Subepicardial in the *crista terminalis* at the junction of the SVC and right atrium [1]. * **Blood Supply:** Most common source is the **Right Coronary Artery (60%)**. Occlusion (as in Inferior Wall MI) often leads to sinus bradycardia. * **Innervation Rule:** **R**ight Vagus → **R**hythm (SA node); **L**eft Vagus → **L**ag/Conduction (AV node) [2]. * **Artery Course:** The nodal artery typically forms an arterial ring (circulus arteriosus) around the termination of the SVC.
Question 263: At which vertebral level does the oesophagus cross the diaphragm?
- A. T8
- B. T10 (Correct Answer)
- C. T12
- D. T2
Explanation: The diaphragm contains three major openings (hiatuses) that allow structures to pass between the thorax and the abdomen. These are high-yield topics for NEET-PG, often remembered by the mnemonic **"Voice Of America"** or **"I Eat 10 Eggs At 12."** ### **Why T10 is Correct** The **oesophageal opening** is located at the level of the **T10** vertebra. It is situated in the muscular part of the right crus of the diaphragm. Apart from the oesophagus, this opening also transmits the anterior and posterior vagal trunks, the oesophageal branches of the left gastric artery, and some lymphatic vessels. ### **Analysis of Incorrect Options** * **T8 (Option A):** This is the level of the **Vena Caval opening**, located in the central tendon. it transmits the Inferior Vena Cava (IVC) and branches of the right phrenic nerve. * **T12 (Option B):** This is the level of the **Aortic opening**, located posterior to the median arcuate ligament. It transmits the Aorta, Azygos vein, and Thoracic duct (Mnemonic: **Red, White, and Blue**). * **T2 (Option D):** This level is in the upper thorax, near the suprasternal notch, far above the diaphragm. ### **Clinical Pearls for NEET-PG** 1. **Muscle Fiber Guarding:** The oesophageal opening is surrounded by fibers of the **right crus**, which act as a physiological sphincter to prevent gastroesophageal reflux. 2. **Hiatal Hernia:** A widening of the T10 opening can lead to a hiatal hernia, where the stomach protrudes into the thoracic cavity. 3. **Summary Table:** * **T8:** IVC (8 letters in "Vena Cava") * **T10:** Oesophagus (10 letters in "Oesophagus") * **T12:** Aortic Hiatus (12 letters in "Aortic Hiatus")
Question 264: Which structure passes through the opening marked 3 in the diaphragm?
- A. Oesophagus
- B. Left gastric nerve
- C. Right phrenic nerve (Correct Answer)
- D. Hemiazygos vein
Explanation: ***Right phrenic nerve*** - The **right phrenic nerve** passes through the **caval foramen** (T8 level), which is opening 3 in the diaphragm diagram. - This opening primarily transmits the **inferior vena cava (IVC)** and the **right phrenic nerve** together. *Oesophagus* - The **oesophagus** passes through the **oesophageal hiatus** at the **T10 level**, not through opening 3. - This hiatus also transmits the **vagal trunks** and **oesophageal branches** of the left gastric vessels. *Left gastric nerve* - The **left gastric nerve** (part of vagal trunk) passes through the **oesophageal hiatus** at **T10**, not opening 3. - It accompanies the **oesophagus** and provides **parasympathetic innervation** to the stomach. *Hemiazygos vein* - The **hemiazygos vein** passes through the **aortic hiatus** at the **T12 level**, not through opening 3. - This hiatus primarily contains the **aorta**, **thoracic duct**, and **azygos/hemiazygos veins**.
Question 265: All of the following are true about coronary arteries, EXCEPT?
- A. Right coronary artery lies in the right anterior coronary sulcus.
- B. Left anterior descending artery is a branch of the left coronary artery.
- C. Usually 3 obtuse marginal arteries arise from the left coronary artery. (Correct Answer)
- D. In 85% of cases, the posterior descending interventricular artery arises from the right coronary artery.
Explanation: ### Explanation **1. Why Option C is the Correct Answer (The False Statement):** The **Obtuse Marginal (OM) arteries** are branches of the **Circumflex artery** (which itself is a branch of the Left Coronary Artery) [1]. While the number of OM branches varies, there are typically **one to three** branches, but they do not arise directly from the main Left Coronary Artery (LCA) trunk [1]. More importantly, the LCA typically divides into only two primary branches: the Left Anterior Descending (LAD) and the Circumflex [1]. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** The **Right Coronary Artery (RCA)** originates from the anterior aortic sinus and runs forward between the pulmonary trunk and the right auricle to settle in the **right anterior coronary sulcus** (atrioventricular groove) [1]. * **Option B:** The **LAD** is indeed one of the two terminal branches of the LCA [1]. It travels in the anterior interventricular groove toward the apex. * **Option C:** **Coronary Dominance** is determined by the origin of the **Posterior Descending Artery (PDA)** [1]. In approximately **85%** of individuals, the PDA arises from the RCA (**Right Dominance**). In 8%, it arises from the Circumflex (Left Dominance), and in 7%, it is Co-dominant. **Clinical Pearls for NEET-PG:** * **LAD** is the most common site of coronary artery occlusion ("The Widow Maker"). * **SA Node Supply:** In 60% of cases, it is supplied by the RCA; in 40%, by the LCA. * **AV Node Supply:** In 90% of cases, it is supplied by the RCA. * **Thebesian Veins:** These are the smallest cardiac veins that drain directly into the heart chambers, mostly the right atrium.
Question 266: The third constriction of the esophagus is at the level of which anatomical landmark?
- A. The crossing of the aorta over the left bronchus
- B. The point where the esophagus pierces the diaphragm (Correct Answer)
- C. The junction of the esophagus and stomach
- D. The cricopharyngeus muscle
Explanation: The esophagus is a muscular tube approximately 25 cm long, characterized by four physiological constrictions. These constrictions are clinically significant as they are common sites for the lodgment of foreign bodies and the development of strictures. **Explanation of the Correct Answer:** The **third constriction** occurs where the esophagus passes through the **diaphragm** at the level of the **T10 vertebra**. This narrowing is caused by the muscular fibers of the right crus of the diaphragm, which act as a physiological sphincter to prevent gastroesophageal reflux [1]. **Analysis of Incorrect Options:** * **Option A:** The crossing of the arch of the aorta and the left principal bronchus represents the **second constriction** (at the level of T4/T5). * **Option C:** The junction of the esophagus and stomach (Gastroesophageal junction) is the site of the anatomical/physiological sphincter but is generally considered the termination point rather than the "third constriction" in standard anatomical numbering [1]. * **Option D:** The cricopharyngeus muscle (at the pharyngoesophageal junction) is the **first constriction**, located 15 cm from the incisor teeth. It is the narrowest part of the entire esophagus. **NEET-PG High-Yield Pearls:** * **Distances from Incisor Teeth:** 1. 1st Constriction (Cricopharynx): 15 cm 2. 2nd Constriction (Aorta/Left Bronchus): 22–25 cm 3. 3rd Constriction (Diaphragm): 37.5–40 cm * **Vertebral Levels:** Starts at C6, pierces diaphragm at T10, and ends at T11. * **Clinical Significance:** These sites are the most common locations for corrosive injury and esophageal carcinoma.
Question 267: A 30-year-old man is diagnosed with a blockage of arterial flow in the proximal part of the thoracic aorta. Brachial arterial pressure is markedly increased, femoral pressure is decreased, and the femoral pulses are delayed. The patient shows no external signs of inflammation. Which of the following structures failed to develop normally?
- A. Second aortic arch
- B. Third aortic arch
- C. Fourth aortic arch (Correct Answer)
- D. Fifth aortic arch
Explanation: **Explanation:** The clinical presentation of hypertension in the upper limbs (increased brachial pressure) and hypotension/delayed pulses in the lower limbs (decreased femoral pressure) is a classic description of **Coarctation of the Aorta (Post-ductal type)**. [1] **Why the Fourth Aortic Arch is Correct:** The adult aorta develops from multiple embryonic sources. Specifically, the **left fourth aortic arch** forms the segment of the **arch of the aorta** between the left common carotid and the left subclavian arteries. A developmental failure or abnormal constriction in this region leads to coarctation. [1] Since the blockage occurs distal to the origin of the great vessels of the head and arms but proximal to the descending aorta, it creates a pressure gradient favoring the upper body. **Analysis of Incorrect Options:** * **A. Second aortic arch:** This arch largely disappears, with its remnants forming the **stapedial** and hyoid arteries. * **B. Third aortic arch:** This arch develops into the **common carotid artery** and the proximal part of the **internal carotid artery**. * **D. Fifth aortic arch:** This is a rudimentary structure that typically regresses completely or fails to form in humans; it does not contribute to the definitive aorta. **High-Yield Clinical Pearls for NEET-PG:** * **Rib Notching:** In coarctation, collateral circulation develops via the intercostal arteries. Pressure erosion on the lower borders of the ribs leads to "rib notching" visible on X-ray. * **Turner Syndrome:** Coarctation of the aorta is the most common cardiac anomaly associated with Turner Syndrome (45, XO). * **Aortic Arch Derivatives:** * 1st: Maxillary artery. * 2nd: Stapedial artery. * 3rd: Carotids. * 4th: Arch of aorta (Left) and R. Subclavian (Right). * 6th: Pulmonary arteries and Ductus arteriosus.
Question 268: The triangle of Koch is bound by all of the following except?
- A. Tendon of Todaro
- B. Septal leaflet of tricuspid valve
- C. AV node (Correct Answer)
- D. Coronary sinus
Explanation: The **Triangle of Koch** is a vital anatomical landmark located in the endocardium of the right atrium. It is used by electrophysiologists to locate the Atrioventricular (AV) node during cardiac procedures [1]. ### **Why Option C is the Correct Answer** The question asks for what does **not** bound the triangle. The **AV node** is the **content** of the triangle, not one of its boundaries. It is located at the apex of the triangle [1]. ### **Explanation of Boundaries (Incorrect Options)** The Triangle of Koch is defined by three specific structures: * **Option A: Tendon of Todaro:** This forms the **superior (posterior-superior)** border [1]. It is a subendocardial continuation of the Eustachian valve (valve of the IVC). * **Option B: Septal leaflet of the tricuspid valve:** This forms the **inferior (anterior-inferior)** border [1]. * **Option D: Coronary sinus:** The opening (os) of the coronary sinus forms the **base** of the triangle [1]. ### **High-Yield Clinical Pearls for NEET-PG** * **Clinical Significance:** The Triangle of Koch is the primary landmark for **catheter ablation** of the slow pathway in AV Nodal Reentrant Tachycardia (AVNRT). * **The Apex:** The point where the Tendon of Todaro meets the tricuspid valve annulus is the apex, which houses the **AV node** [1]. * **The Base:** Formed by the Thebesian valve (valve of the coronary sinus). * **Mnemonics:** Remember **"T-S-C"** for boundaries: **T**odaro, **S**eptal leaflet, **C**oronary sinus. * **Surgical Note:** During atrial septal defect (ASD) repairs, surgeons must identify this triangle to avoid placing sutures in the AV node, which would cause a complete heart block [1].
Question 269: In a standing man, at the midaxillary line, where does the lower border of the pleura reach?
- A. 6th rib
- B. 8th rib
- C. 10th rib (Correct Answer)
- D. 12th rib
Explanation: ### Explanation The surface anatomy of the lungs and pleura is a high-yield topic for NEET-PG, specifically the relationship between the lower borders and the thoracic cage. **1. Why the 10th Rib is Correct:** The lower border of the **pleura** (the costodiaphragmatic reflection) follows an "even number" rule (8, 10, 12) across three vertical lines: * **Midclavicular line:** 8th rib * **Midaxillary line:** 10th rib (Correct Answer) * **Scapular line/Paravertebral:** 12th rib **2. Analysis of Incorrect Options:** * **6th rib (Option A):** This represents the lower border of the **lung** at the midclavicular line. * **8th rib (Option B):** This represents the lower border of the **lung** at the midaxillary line, or the lower border of the **pleura** at the midclavicular line. * **12th rib (Option D):** This represents the lower border of the **pleura** at the scapular line or posteriorly. **3. Clinical Pearls & High-Yield Facts:** * **The "Rule of Two":** The pleura always extends approximately two ribs lower than the lung at any given vertical line. This creates the **costodiaphragmatic recess**, a potential space where fluid (pleural effusion) first accumulates [1]. * **Thoracocentesis (Pleural Tap):** To avoid injuring the lung, the needle is typically inserted in the midaxillary line, one or two intercostal spaces below the lung border but above the pleural border (usually the 8th or 9th intercostal space) [1]. * **Safety Tip:** Always insert the needle at the **upper border of the lower rib** to avoid damaging the neurovascular bundle (VAN) located in the costal groove [1].
Question 270: Which of the following statements about the esophageal hiatus is TRUE?
- A. It lies between the two cura of the diaphragm.
- B. It is located at the level of the D12 vertebra.
- C. The vagus nerve passes through it. (Correct Answer)
- D. All of the above.
Explanation: The esophageal hiatus is a vital opening in the diaphragm, and understanding its anatomy is high-yield for NEET-PG. ### **Explanation of the Correct Answer** **Option C is correct.** The esophageal hiatus (located at the **T10 level**) transmits the esophagus along with the **anterior and posterior vagal trunks** [2] (derived from the left and right vagus nerves, respectively). Additionally, it transmits the esophageal branches of the left gastric vessels and a few lymphatics. ### **Analysis of Incorrect Options** * **Option A:** The esophageal hiatus is not located between the two crura; rather, it is formed by the **splitting of the fibers of the right crus** of the diaphragm. This anatomical arrangement acts as a "pinch-cock" sphincter, preventing gastroesophageal reflux during inspiration [3]. * **Option B:** The esophageal hiatus is located at the level of the **10th thoracic vertebra (T10)**. The T12 level corresponds to the **Aortic hiatus**, which lies posterior to the median arcuate ligament. ### **High-Yield NEET-PG Pearls** To remember the levels of the major diaphragmatic openings, use the mnemonic **"I Eat 10 Eggs At 12"**: 1. **I (IVC):** T8 level (passes through the central tendon). 2. **Eat (Esophagus):** T10 level (formed by the right crus). 3. **At (Aorta):** T12 level (transmits the Aorta, Azygos vein, and Thoracic duct—mnemonic: **red, white, and blue**). **Clinical Correlation:** A sliding hiatal hernia occurs when the gastroesophageal junction protrudes through the esophageal hiatus into the posterior mediastinum, often due to the widening of the right crus [1, 2].
Practice by Chapter
Thoracic Wall and Diaphragm
Practice Questions
Pleura and Lungs
Practice Questions
Mediastinum
Practice Questions
Heart and Pericardium
Practice Questions
Great Vessels and Azygos System
Practice Questions
Thoracic Duct and Lymphatics
Practice Questions
Autonomic Innervation
Practice Questions
Applied Anatomy and Clinical Correlations
Practice Questions
Thoracic Imaging and Cross-sectional Anatomy
Practice Questions
Embryological Development of Thoracic Structures
Practice Questions
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free