Surgery MCQs Q20

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SURGERY • ANATOMY

A 40-year-old woman undergoes a radical mastectomy. Postoperatively, she is unable to place her right hand behind her back or reach the opposite scapula.

Which nerve is most likely injured?

A Medial pectoral nerve

B Long thoracic nerve

C Thoracodorsal nerve

D Subscapular nerve

E Suprascapular nerve

Thoracodorsal nerve. The inability to reach behind the back or to the opposite scapula points to a paralysis of the Latissimus Dorsi, which is supplied by the Thoracodorsal nerve.

Decoding Clue

1

CLUE "Post-mastectomy + inability to reach back/opposite scapula"

TRANS Loss of extension, adduction, and medial rotation → Thoracodorsal Nerve (Latissimus dorsi).

Explanation

C. Thoracodorsal nerve: Correct. Innervates the latissimus dorsi. This muscle extends, adducts, and medially rotates the humerus. Injury (common in axillary clearance) makes it impossible to reach the back pockets or opposite shoulder blade.

A. Medial pectoral: Supplies Pectoralis minor and part of Pectoralis major. Affects adduction and medial rotation, but isn't the primary "reaching back" muscle.

B. Long thoracic: Supplies Serratus anterior. Injury causes "winged scapula" and inability to abduct above the horizontal plane.

D. Subscapular: Supplies Subscapularis (medial rotation) and Teres major. While Teres major assists the Lat dorsi, the profound deficit described is classic for Thoracodorsal injury.

E. Suprascapular: Supplies Supraspinatus and Infraspinatus (abduction initiation and external rotation).

🧠 High-Yield Pearls

Nerve	Muscle	Clinical Deficit
Thoracodorsal	Latissimus dorsi	Weak arm extension, adduction, medial rotation (reaching back)
Long Thoracic	Serratus anterior	Winged scapula; difficulty with overhead reaching
Axillary	Deltoid	Inability to abduct arm from 15° to 90°
Suprascapular	Supraspinatus	Inability to initiate abduction (first 15°)

Integrated Clinical Questions

1. Nerve injured causing winged scapula?

Answer: Long thoracic nerve (supplying Serratus anterior).

2. Muscle responsible for initiating abduction?

Answer: Supraspinatus (first 15 degrees).

3. Main action of latissimus dorsi?

Answer: Extension, adduction, and medial rotation of the humerus (the "climbing" muscle).

4. Which nerve runs with the subscapular vessels?

Answer: Thoracodorsal nerve.

5. Why is the thoracodorsal nerve vulnerable during mastectomy?

Answer: It is located in the axillary fat that is typically removed during axillary lymph node dissection.

⚡ Exam Pearls

• Reaching behind back = Latissimus dorsi function.
• Lat dorsi = Thoracodorsal nerve (C6, C7, C8).
• Winged scapula = Long thoracic nerve.
• Axillary dissection: Always watch out for Thoracodorsal and Long Thoracic nerves.

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