COMSAE Form 108B: Practice Questions with
Answers and Rationales
,Section 1: Anatomy and Embryology
1. A 6-year-old boy presents with recurrent respiratory infections, chronic diarrhea, and failure to
thrive. Laboratory studies reveal absent thymic shadow and markedly decreased T-cell counts.
Which embryologic structure most likely failed to develop properly?
A. First pharyngeal pouch
B. Second pharyngeal arch
C. Third and fourth pharyngeal pouches
D. Neural crest cells of the forebrain
Correct Answer: C. Third and fourth pharyngeal pouches
Rationale: The thymus and inferior parathyroid glands develop from the third pharyngeal pouch,
while the superior parathyroids arise from the fourth pouch. Failure of development leads to
DiGeorge syndrome with T-cell deficiency and recurrent infections .
,2. Which vessel supplies the sinoatrial node in most individuals?
A. Left anterior descending artery
B. Right coronary artery
C. Circumflex artery
D. Posterior descending artery
Correct Answer: B. Right coronary artery
Rationale: The sinoatrial node receives blood supply from the right coronary artery in
approximately 60% of individuals, making it the most common source .
, 3. Which cranial nerve innervates the lateral rectus muscle?
A. CN III (Oculomotor)
B. CN IV (Trochlear)
C. CN VI (Abducens)
D. CN V (Trigeminal)
Correct Answer: C. CN VI (Abducens)
Rationale: The abducens nerve innervates the lateral rectus muscle, which abducts the eye. CN III
innervates most extraocular muscles, and CN IV innervates the superior oblique .
4. A 60-year-old man with chronic smoking history develops hematuria. Histology reveals
transitional cell carcinoma. Which structure is lined by transitional epithelium?
Answers and Rationales
,Section 1: Anatomy and Embryology
1. A 6-year-old boy presents with recurrent respiratory infections, chronic diarrhea, and failure to
thrive. Laboratory studies reveal absent thymic shadow and markedly decreased T-cell counts.
Which embryologic structure most likely failed to develop properly?
A. First pharyngeal pouch
B. Second pharyngeal arch
C. Third and fourth pharyngeal pouches
D. Neural crest cells of the forebrain
Correct Answer: C. Third and fourth pharyngeal pouches
Rationale: The thymus and inferior parathyroid glands develop from the third pharyngeal pouch,
while the superior parathyroids arise from the fourth pouch. Failure of development leads to
DiGeorge syndrome with T-cell deficiency and recurrent infections .
,2. Which vessel supplies the sinoatrial node in most individuals?
A. Left anterior descending artery
B. Right coronary artery
C. Circumflex artery
D. Posterior descending artery
Correct Answer: B. Right coronary artery
Rationale: The sinoatrial node receives blood supply from the right coronary artery in
approximately 60% of individuals, making it the most common source .
, 3. Which cranial nerve innervates the lateral rectus muscle?
A. CN III (Oculomotor)
B. CN IV (Trochlear)
C. CN VI (Abducens)
D. CN V (Trigeminal)
Correct Answer: C. CN VI (Abducens)
Rationale: The abducens nerve innervates the lateral rectus muscle, which abducts the eye. CN III
innervates most extraocular muscles, and CN IV innervates the superior oblique .
4. A 60-year-old man with chronic smoking history develops hematuria. Histology reveals
transitional cell carcinoma. Which structure is lined by transitional epithelium?
