2026 Version COMSAE Phase 3 Practice
Exam Questions with Answers and
Detailed Rationales
Question 1
A 67-year-old man with a 45-pack-year smoking history
presents with progressive dyspnea, chronic cough, and
weight loss. CT of the chest reveals a 3.5-cm central hilar
mass. Serum sodium is 121 mEq/L. Laboratory studies
demonstrate low serum osmolality and inappropriately
concentrated urine. Physical examination shows no
peripheral edema. Which of the following is the most likely
underlying mechanism of this patient's electrolyte
abnormality?
A. Increased secretion of aldosterone
B. Increased renal sodium reabsorption
C. Ectopic secretion of antidiuretic hormone
D. Decreased secretion of atrial natriuretic peptide
Answer: C. Ectopic secretion of antidiuretic hormone
Rationale: The patient has a central lung mass, likely small-
cell carcinoma, associated with ectopic ADH secretion. Excess
ADH causes increased water reabsorption in the collecting
ducts, producing dilutional hyponatremia, low serum
osmolality, and inappropriately concentrated urine. This is the
classic presentation of SIADH. Aldosterone primarily regulates
,sodium and potassium balance and would not produce this
pattern of concentrated urine with hypotonic hyponatremia.
Question 2
A 28-year-old woman presents with fatigue, weight gain,
constipation, and cold intolerance. Laboratory studies reveal
elevated TSH and low free T4. She has a painless, diffusely
enlarged thyroid. Which histopathologic finding is most
likely?
A. Noncaseating granulomas
B. Papillary projections with fibrovascular cores
C. Lymphocytic infiltration with germinal centers
D. Follicular cells containing colloid surrounded by a fibrous
capsule
Answer: C. Lymphocytic infiltration with germinal centers
Rationale: Hashimoto thyroiditis is the most common cause of
hypothyroidism in iodine-sufficient regions. It is characterized
by autoimmune destruction of thyroid tissue, lymphocytic
infiltration, germinal centers, and eventual thyroid atrophy.
Anti-thyroid peroxidase antibodies are commonly present.
Papillary projections suggest papillary thyroid carcinoma,
whereas noncaseating granulomas are associated with
sarcoidosis.
Question 3
,A 59-year-old man develops sudden right-sided weakness and
aphasia. He has a history of poorly controlled hypertension
and atrial fibrillation. CT demonstrates an ischemic infarct
involving the dominant hemisphere. Which vascular territory
is most likely affected?
A. Right posterior cerebral artery
B. Left posterior cerebral artery
C. Left middle cerebral artery
D. Left anterior cerebral artery
Answer: C. Left middle cerebral artery
Rationale: A dominant-hemisphere MCA infarction classically
causes contralateral face and upper-extremity weakness
greater than lower-extremity weakness, along with aphasia.
The dominant hemisphere is usually the left hemisphere. The
anterior cerebral artery primarily supplies the medial frontal
and parietal lobes and causes predominant contralateral
lower-extremity weakness.
Question 4
A 42-year-old woman presents with severe epigastric pain
radiating to the back. Serum lipase is markedly elevated. She
reports recurrent episodes after consuming large meals.
Ultrasound demonstrates multiple gallstones. Which
mechanism most directly explains the pancreatic injury?
A. Autoimmune destruction of pancreatic acinar cells
B. Premature activation of trypsinogen within pancreatic
acinar cells
, C. Increased bicarbonate secretion from pancreatic ducts
D. Decreased secretion of cholecystokinin
Answer: B. Premature activation of trypsinogen within
pancreatic acinar cells
Rationale: Acute pancreatitis involves inappropriate
activation of pancreatic digestive enzymes within the
pancreas. Trypsinogen is converted to trypsin, which activates
other enzymes, leading to autodigestion and inflammation.
Gallstones are a major cause because obstruction at the
ampulla of Vater can increase pancreatic ductal pressure and
promote enzyme activation.
Question 5
A 34-year-old man presents with fever, hypotension, diffuse
abdominal pain, and rigid abdominal muscles. CT
demonstrates free intraperitoneal air. He undergoes
emergency surgery for perforated bowel. Several hours later,
he develops worsening hypotension despite aggressive fluid
administration. Which mediator is most responsible for the
systemic vasodilation associated with his condition?
A. Histamine
B. Thromboxane A2
C. Nitric oxide
D. Endothelin-1
Answer: C. Nitric oxide
Exam Questions with Answers and
Detailed Rationales
Question 1
A 67-year-old man with a 45-pack-year smoking history
presents with progressive dyspnea, chronic cough, and
weight loss. CT of the chest reveals a 3.5-cm central hilar
mass. Serum sodium is 121 mEq/L. Laboratory studies
demonstrate low serum osmolality and inappropriately
concentrated urine. Physical examination shows no
peripheral edema. Which of the following is the most likely
underlying mechanism of this patient's electrolyte
abnormality?
A. Increased secretion of aldosterone
B. Increased renal sodium reabsorption
C. Ectopic secretion of antidiuretic hormone
D. Decreased secretion of atrial natriuretic peptide
Answer: C. Ectopic secretion of antidiuretic hormone
Rationale: The patient has a central lung mass, likely small-
cell carcinoma, associated with ectopic ADH secretion. Excess
ADH causes increased water reabsorption in the collecting
ducts, producing dilutional hyponatremia, low serum
osmolality, and inappropriately concentrated urine. This is the
classic presentation of SIADH. Aldosterone primarily regulates
,sodium and potassium balance and would not produce this
pattern of concentrated urine with hypotonic hyponatremia.
Question 2
A 28-year-old woman presents with fatigue, weight gain,
constipation, and cold intolerance. Laboratory studies reveal
elevated TSH and low free T4. She has a painless, diffusely
enlarged thyroid. Which histopathologic finding is most
likely?
A. Noncaseating granulomas
B. Papillary projections with fibrovascular cores
C. Lymphocytic infiltration with germinal centers
D. Follicular cells containing colloid surrounded by a fibrous
capsule
Answer: C. Lymphocytic infiltration with germinal centers
Rationale: Hashimoto thyroiditis is the most common cause of
hypothyroidism in iodine-sufficient regions. It is characterized
by autoimmune destruction of thyroid tissue, lymphocytic
infiltration, germinal centers, and eventual thyroid atrophy.
Anti-thyroid peroxidase antibodies are commonly present.
Papillary projections suggest papillary thyroid carcinoma,
whereas noncaseating granulomas are associated with
sarcoidosis.
Question 3
,A 59-year-old man develops sudden right-sided weakness and
aphasia. He has a history of poorly controlled hypertension
and atrial fibrillation. CT demonstrates an ischemic infarct
involving the dominant hemisphere. Which vascular territory
is most likely affected?
A. Right posterior cerebral artery
B. Left posterior cerebral artery
C. Left middle cerebral artery
D. Left anterior cerebral artery
Answer: C. Left middle cerebral artery
Rationale: A dominant-hemisphere MCA infarction classically
causes contralateral face and upper-extremity weakness
greater than lower-extremity weakness, along with aphasia.
The dominant hemisphere is usually the left hemisphere. The
anterior cerebral artery primarily supplies the medial frontal
and parietal lobes and causes predominant contralateral
lower-extremity weakness.
Question 4
A 42-year-old woman presents with severe epigastric pain
radiating to the back. Serum lipase is markedly elevated. She
reports recurrent episodes after consuming large meals.
Ultrasound demonstrates multiple gallstones. Which
mechanism most directly explains the pancreatic injury?
A. Autoimmune destruction of pancreatic acinar cells
B. Premature activation of trypsinogen within pancreatic
acinar cells
, C. Increased bicarbonate secretion from pancreatic ducts
D. Decreased secretion of cholecystokinin
Answer: B. Premature activation of trypsinogen within
pancreatic acinar cells
Rationale: Acute pancreatitis involves inappropriate
activation of pancreatic digestive enzymes within the
pancreas. Trypsinogen is converted to trypsin, which activates
other enzymes, leading to autodigestion and inflammation.
Gallstones are a major cause because obstruction at the
ampulla of Vater can increase pancreatic ductal pressure and
promote enzyme activation.
Question 5
A 34-year-old man presents with fever, hypotension, diffuse
abdominal pain, and rigid abdominal muscles. CT
demonstrates free intraperitoneal air. He undergoes
emergency surgery for perforated bowel. Several hours later,
he develops worsening hypotension despite aggressive fluid
administration. Which mediator is most responsible for the
systemic vasodilation associated with his condition?
A. Histamine
B. Thromboxane A2
C. Nitric oxide
D. Endothelin-1
Answer: C. Nitric oxide
