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Question 2
Anatomy → Region
Classification
Question
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During a transverse incision through the lower anterior abdominal wall, the surgeon identifies only two distinct muscular layers at the lateral margin of the incision, rather than the three layers expected more superiorly. Which anatomical relationship best explains this finding?
Question Stem
A) External oblique fuses with the rectus sheath
B) Internal oblique fuses with external oblique
C) Transversus abdominis fuses with internal oblique
D) Rectus abdominis fuses with transversus abdominis
E) Pyramidalis fuses with internal oblique
B) Internal oblique fuses with external oblique
C) Transversus abdominis fuses with internal oblique
D) Rectus abdominis fuses with transversus abdominis
E) Pyramidalis fuses with internal oblique
Options
A
External oblique fuses with the rectus sheath
B
Internal oblique fuses with external oblique
C
Transversus abdominis fuses with internal oblique
D
Rectus abdominis fuses with transversus abdominis
E
Pyramidalis fuses with internal oblique
Explanation
The lateral abdominal wall normally contains external oblique, internal oblique and transversus abdominis. In the lower abdomen, however, the lower portion of transversus abdominis becomes fused with internal oblique. Consequently, during a transverse lower abdominal incision, only two distinct muscular layers may be discernible at the lateral margin. This is an important applied anatomical relationship when interpreting the layers encountered during pelvic and lower abdominal surgery. External oblique remains a separate superficial flank muscle and does not fuse with the rectus sheath in the manner described. Rectus abdominis is a vertically orientated anterior abdominal wall muscle contained within the rectus sheath, while pyramidalis is another vertical muscle and does not account for the reduced number of lateral muscle layers. The operative finding therefore reflects fusion of the lower transversus abdominis with internal oblique.
Option Validity
A) External oblique does not account for the reduction to two discernible lateral muscle layers.
B) The relevant lower abdominal fusion is not between internal and external oblique.
D) Rectus abdominis is a vertical muscle and does not fuse with transversus abdominis in this relationship.
E) Pyramidalis does not produce the described lateral abdominal-wall arrangement.
Further Reading
Part 1 MRCOG Revision Notes and Sample SBAs. Cambridge University Press, 2020. Section 1: Anatomy, Chapter 1. https://doi.org/10.1017/9781108644396