Thorax — MCQs

Thorax Indian Medical PG Practice Questions and MCQs

Question 651: Sympathetic supply to the heart arises from which of the following spinal segments?

A. T1 to T5 (Correct Answer)
B. T2 to T6
C. T3 to T7
D. T4 to T8

Explanation: The preganglionic sympathetic fibers that innervate the heart originate from the lateral horns of the thoracic spinal segments T1 to T5. These fibers synapse in the cervical and upper thoracic sympathetic ganglia, from which postganglionic fibers extend to the heart. While there is some overlap, the primary and most significant sympathetic innervation to the heart stems predominantly from T1 to T5, making T2 to T6 a less precise answer. Including T6 would extend past the typical primary cardiac sympathetic innervation, which largely concludes at T5. This range is too caudal and largely beyond the principal segments providing sympathetic innervation to the heart. Segments T6-T8 are more involved in sympathetic supply to abdominal organs and other structures rather than direct cardiac control.

Question 652: Which of the following is not a boundary of Koch's triangle?

A. Limbus fossa ovalis (Correct Answer)
B. Tricuspid valve ring
C. Coronary sinus
D. Tendon of Todaro

Explanation: ***Limbus fossa ovalis*** - The **limbus fossa ovalis** is a prominent oval ridge on the **interatrial septum** that surrounds the fossa ovalis. - It is **not involved** in forming the boundaries of Koch's triangle, which is located in the **right atrium** near the AV node [1]. *Tricuspid valve ring* - The **tricuspid valve ring** (or annulus) forms one of the key anatomical boundaries of **Koch's triangle**, specifically its base [1]. - This **fibrous ring** anchors the tricuspid valve leaflets and marks the inferior aspect of the triangle [1]. *Coronary sinus* - The **coronary sinus ostium** (opening) forms another crucial boundary of **Koch's triangle** [1]. - It is located at the **inferior-posterior aspect** of the interatrial septum, opening into the right atrium [1]. *Tendon of todaro* - The **Tendon of Todaro** is a fibrous structure that forms the superior boundary of **Koch's triangle** [1]. - It extends from the **Eustachian valve** (of the inferior vena cava) towards the central fibrous body, playing a role in **AV nodal localization** [1].

Question 653: Azygos vein drains into:

A. Left brachiocephalic vein
B. Inferior vena cava
C. Superior vena cava (Correct Answer)
D. Right brachiocephalic vein

Explanation: Wait, what? Azygos vein drains into: ***Superior vena cava*** - The **azygos vein** ascends along the right side of the vertebral column and typically arches over the root of the right lung before draining into the **superior vena cava (SVC)**. - This anatomical arrangement allows the azygos system to collect venous blood from the posterior thoracic and abdominal walls, as well as the bronchi and esophagus, ultimately returning it to the systemic circulation via the SVC [1]. *Left brachiocephalic vein* - The **left brachiocephalic vein** drains blood from the upper left limb and left side of the head and neck. - It merges with the right brachiocephalic vein to form the SVC; the azygos vein does not directly drain into it. *Inferior vena cava* - The **inferior vena cava (IVC)** collects deoxygenated blood from the lower body. - The azygos system primarily drains structures above the diaphragm, distinct from the IVC's drainage area. *Right brachiocephalic vein* - The **right brachiocephalic vein** drains blood from the upper right limb and right side of the head and neck. - While it contributes to the formation of the SVC, the azygos vein's direct connection is to the SVC itself, not the right brachiocephalic vein.

Question 654: The thoracic duct crosses from the right to the left at the level of

A. T12 vertebra
B. T2 vertebra
C. T4-T5 vertebra (Correct Answer)
D. T6 vertebra

Explanation: ***T4-T5 vertebra*** - The **thoracic duct** crosses from the right to the left side of the vertebral column at the level of the **T4-T5 vertebrae**, specifically just above the root of the left lung. - This crossover is an important anatomical landmark as it signifies the duct's ascent towards the neck to drain into the left subclavian vein. *T12 vertebra* - The **thoracic duct** originates from the **cisterna chyli** at the level of the L1 or L2 vertebra and ascends into the thorax at or below the T12 vertebra, it does not cross over at this level. - This level primarily marks its entry into the thoracic cavity, not its main crossover point. *T6 vertebra* - While the **thoracic duct** is present in the thorax at this level, it does not undergo its characteristic crossover from right to left at the T6 vertebra. - The duct continues its ascent along the right side of the vertebral column before moving across. *T2 vertebra* - By the level of the T2 vertebra, the **thoracic duct** has already crossed to the left side of the vertebral column and is ascending towards its termination in the neck. - The crossover event occurs more inferiorly, at the T4-T5 level.

Question 655: Which of the following muscles is considered the most powerful elevator of the ribs?

A. Levatores costarum
B. External intercostals (Correct Answer)
C. Serratus posterior inferior
D. Serratus posterior superior

Explanation: ***External intercostals*** - The **external intercostals** are the **most powerful and primary elevators of the ribs** during inspiration [1]. - Their fibers run **obliquely downward and forward** from the rib above to the rib below, providing optimal mechanical advantage for rib elevation [1]. - They are the **principal muscles of normal inspiration**, actively contracting to elevate the ribs and increase the anteroposterior and lateral dimensions of the thoracic cavity [1]. - Their action is continuous during normal breathing, making them the most significant contributors to rib elevation. *Levatores costarum* - The **levatores costarum** are small muscles originating from transverse processes of vertebrae and inserting into the rib below. - While they do elevate ribs, they are considered **accessory muscles of inspiration** with a minor role. - Their primary function may be more related to **spinal stabilization** than powerful rib elevation. - They are not as powerful or mechanically effective as the external intercostals. *Serratus posterior inferior* - The **serratus posterior inferior** muscles primarily function to **depress the lower ribs**, aiding in forced expiration. - Their action is antagonistic to rib elevation and thus they are not involved in raising the ribs. *Serratus posterior superior* - The **serratus posterior superior** muscles assist in **elevating the upper ribs** during inspiration. - They are **accessory muscles of inspiration** but play a minor role compared to the external intercostals. - Their action is limited to the upper ribs (ribs 2-5) and they are not continuously active during normal breathing.

Question 656: All of the following are true about Sibson's fascia except which of the following?

A. Subclavian artery passes beneath it.
B. Attached to cervical pleura
C. Arises from scalenus anterior (Correct Answer)
D. It forms the suprapleural membrane

Explanation: ***Arises from scalenus anterior*** - This statement is **FALSE** and is the correct answer to this "EXCEPT" question. - Sibson's fascia (suprapleural membrane) does **NOT** arise from the scalenus anterior muscle. - It is a fibrous extension of the **deep cervical fascia** that arises primarily from the **transverse process of the 7th cervical vertebra (C7)** and extends to attach to the **inner border of the first rib**. - While it may have fascial connections with scalene muscles (particularly scalenus medius and minimus), it does not directly arise from scalenus anterior. *Subclavian artery passes beneath it* - This statement is **TRUE**. - The subclavian artery passes through the thoracic inlet and courses beneath (inferior to) the dome-shaped Sibson's fascia. - Sibson's fascia forms a protective dome over the lung apex, and the subclavian vessels pass below this fascial layer in the root of the neck. *Attached to cervical pleura* - This statement is **TRUE**. - Sibson's fascia is firmly **attached to the cervical pleura** (the pleura covering the apex of the lung). - This attachment reinforces and supports the pleural dome, preventing herniation of the lung apex into the neck during respiratory movements. *It forms the suprapleural membrane* - This statement is **TRUE**. - Sibson's fascia is synonymous with the **suprapleural membrane**. - This membrane strengthens the parietal pleura at the thoracic inlet and maintains the integrity of the pleural cavity at its superior aspect [1].

Question 657: All of the following attach to the first rib, EXCEPT:

A. Scalenus medius
B. Suprapleural membrane
C. Scalenus anterior
D. Scalenus posterior (Correct Answer)

Explanation: ***Scalenus posterior*** - The **scalenus posterior** muscle typically inserts onto the **second rib**, not the first rib. - Its primary actions involve elevating the second rib and flexing the neck laterally. *Scalenus anterior* - The **scalenus anterior** muscle originates from the cervical vertebrae and inserts onto the **first rib**. - It helps elevate the first rib during inspiration and flex the neck. *Scalenus medius* - The **scalenus medius** muscle also originates from the cervical vertebrae and inserts onto the **first rib**, posterior to the scalenus anterior. - It also aids in elevating the first rib and flexing the neck. *Suprapleural membrane* - The **suprapleural membrane**, also known as **Sibson's fascia**, is a fibrous sheet that covers the apex of the lung and is attached to the inner border of the **first rib**. - It acts as a protective dome over the lung apex.

Question 658: Which of the following does not form the left border of the heart?

A. Pulmonary veins (Correct Answer)
B. Left ventricle
C. Aortic arch
D. Pulmonary trunk

Explanation: **Pulmonary veins** - The pulmonary veins are positioned posteriorly and drain into the **left atrium**, thus they do not form part of the visible left cardiac border on a standard chest X-ray or during superficial anatomical viewing [1]. - The **left border of the heart** is primarily formed by the left ventricle, with contributions from the aortic arch and pulmonary trunk more superiorly [3]. *Left ventricle* - The **left ventricle** constitutes the major part of the left border of the heart, extending from the base to the apex [4]. - Its location and size make it a prominent feature on the left silhouette. *Aortic arch* - The **aortic arch** is located superior to the heart and gives rise to major systemic arteries [2]. - It forms a portion of the **upper left border** of the mediastinal silhouette, contributing to the cardiovascular outline. *Pulmonary trunk* - The **pulmonary trunk** arises from the right ventricle and branches into the pulmonary arteries [3]. - It contributes to the **upper left border** of the heart, medial to the aortic arch, as it courses superiorly before bifurcating.

Question 659: Intercostal veins drain into which structure?

A. Left subclavian vein
B. Internal jugular vein
C. Inferior vena cava
D. Azygos vein (Correct Answer)

Explanation: ***Azygos vein*** - The **posterior intercostal veins** drain into the **azygos system (azygos, hemiazygos, and accessory hemiazygos veins)**, which then empties into the superior vena cava [1]. - The azygos vein is centrally located in the posterior mediastinum and serves as a major venous drainage pathway for the thoracic wall [1]. *Left subclavian vein* - The left subclavian vein primarily drains the **left upper limb**. - It receives the **thoracic duct** and occasionally the left external jugular vein, but not the intercostal veins directly. *Internal jugular vein* - The internal jugular vein drains blood from the **brain, superficial face, and neck**. - It joins with the subclavian vein to form the brachiocephalic vein, but does not receive direct intercostal drainage. *Inferior vena cava* - The inferior vena cava drains deoxygenated blood from the **lower body, including the legs, kidneys, and liver**. - It does not directly receive intercostal veins, which primarily drain into the azygos system or brachiocephalic veins.

Question 660: While performing drainage of fluid from the pleural cavity, the needle is introduced through all of the following structures except which one?

A. Thoracic fascia
B. Skin
C. Pulmonary pleura (Correct Answer)
D. Intercostal muscles

Explanation: ***Pulmonary pleura*** - The needle for pleural fluid drainage, or thoracentesis, passes through the **parietal pleura** [1] but not the **pulmonary (visceral) pleura**. - Puncturing the pulmonary pleura would indicate the needle has entered the lung parenchyma, which is an avoidable complication. *Skin* - The **skin** is the outermost layer and the first structure the needle penetrates during a thoracentesis. - It must be sterilized prior to the procedure. *Thoracic fascia* - The needle passes through the **superficial fascia** and then the **deep fascia** covering the intercostal muscles. - These fascial layers provide structural support and enclose the musculature of the thoracic wall. *Intercostal muscles* - The needle traverses the **external**, **internal**, and **innermost intercostal muscles** as it moves deeper into the thoracic cavity. - The neurovascular bundle runs between the internal and innermost intercostals, hence the needle is typically inserted over the superior border of the rib to avoid it [1].

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Thoracic Wall and Diaphragm

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Pleura and Lungs

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Mediastinum

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Heart and Pericardium

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Great Vessels and Azygos System

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Thoracic Duct and Lymphatics

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Autonomic Innervation

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Applied Anatomy and Clinical Correlations

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Thoracic Imaging and Cross-sectional Anatomy

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Embryological Development of Thoracic Structures

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Thorax — MCQs

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