USMLE Step 1 First Aid – 35th Edition
Questions and Answers (Accurate) Rated A+
1. A 6-month-old infant presents with poor feeding, vomiting, and
lethargy. Labs show hypoglycemia, hyperammonemia, and elevated
orotic acid. Which enzyme deficiency is most likely responsible?
A. Carbamoyl phosphate synthetase I
B. Ornithine transcarbamylase
C. Argininosuccinate synthetase
D. Arginase
Correct answer: B. Ornithine transcarbamylase deficiency leads to
excess carbamoyl phosphate, which is shunted into pyrimidine
synthesis causing elevated orotic acid. It is X-linked and commonly
presents with hyperammonemia. Hypoglycemia and vomiting are
common early signs.
2. A patient develops flushing, wheezing, and diarrhea after eating
certain foods. Which substance is most directly responsible?
A. Histamine
B. Bradykinin
C. Serotonin
D. Prostaglandin E2
Correct answer: C. Serotonin is secreted by carcinoid tumors and
causes flushing, diarrhea, and bronchospasm. It is derived from
tryptophan. Chronic excess can lead to niacin deficiency.
3. A drug that inhibits dihydrofolate reductase would most directly impair
synthesis of which molecule?
A. Thymidine
B. Cytosine
C. Guanine
D. Adenine
Correct answer: A. Dihydrofolate reductase is required for regeneration
of tetrahydrofolate, which is essential for thymidine synthesis.
Inhibition leads to impaired DNA replication. Rapidly dividing cells are
most affected.
4. A patient has loss of pain and temperature sensation on the right side
of the body and left side of the face. Where is the lesion?
A. Pons
B. Medulla
C. Midbrain
D. Cervical spinal cord
Correct answer: B. This pattern is classic for lateral medullary
(Wallenberg) syndrome. It involves damage to spinothalamic and
trigeminal pathways. The most common cause is PICA infarction.
5. A newborn has a continuous machine-like heart murmur. Which drug
promotes closure of the responsible structure?
, A. Alprostadil
B. Indomethacin
C. Nitric oxide
D. Sildenafil
Correct answer: B. Indomethacin inhibits prostaglandin synthesis,
promoting closure of the ductus arteriosus. A PDA causes a continuous
murmur. Prostaglandins normally keep the ductus open.
6. A patient with chronic kidney disease develops secondary
hyperparathyroidism. What is the primary mechanism?
A. Increased calcitriol production
B. Decreased phosphate excretion
C. Increased calcium absorption
D. Increased vitamin D activation
Correct answer: B. Reduced phosphate excretion leads to
hyperphosphatemia, which lowers serum calcium. This stimulates
parathyroid hormone release. Low calcitriol further worsens
hypocalcemia.
7. Which immunoglobulin is most important for mucosal immunity?
A. IgG
B. IgA
C. IgM
D. IgE
Correct answer: B. IgA is the predominant antibody in mucosal
secretions such as saliva and breast milk. It exists as a dimer with a
secretory component. It prevents pathogen adherence.
8. A tumor suppressor gene typically functions by which mechanism?
A. Promoting cell division
B. Activating oncogenes
C. Inhibiting cell cycle progression
D. Increasing telomerase activity
Correct answer: C. Tumor suppressor genes slow cell division or
promote apoptosis. Loss of function mutations lead to uncontrolled
growth. Both alleles usually must be inactivated.
9. A patient with Graves disease has elevated thyroid hormone levels.
Which finding is expected?
A. Increased TSH
B. Decreased T3 and T4
C. Increased radioactive iodine uptake
D. Bradycardia
Correct answer: C. Graves disease causes diffuse thyroid hyperactivity,
increasing iodine uptake. TSH is suppressed due to negative feedback.
Patients typically have tachycardia and weight loss.
10. Which vitamin deficiency causes impaired night vision?
A. Vitamin A
B. Vitamin D
C. Vitamin K
, D. Vitamin C
Correct answer: A. Vitamin A is required for rhodopsin formation in
retinal rods. Deficiency leads to night blindness. Severe cases may
cause xerophthalmia.
11. A patient presents with muscle weakness that improves with
repeated use. Which antibody is involved?
A. Acetylcholine receptor
B. Voltage-gated calcium channel
C. Sodium channel
D. Potassium channel
Correct answer: B. Lambert-Eaton syndrome involves antibodies
against presynaptic calcium channels. Repeated use increases calcium
buildup, improving strength. It is often paraneoplastic.
12. Which organism lacks a cell wall?
A. Streptococcus pneumoniae
B. Mycoplasma pneumoniae
C. Escherichia coli
D. Staphylococcus aureus
Correct answer: B. Mycoplasma lacks a peptidoglycan cell wall. This
makes beta-lactam antibiotics ineffective. It is associated with atypical
pneumonia.
13. A mutation causing a frameshift is most likely due to what
change?
A. Single nucleotide substitution
B. Deletion not divisible by three
C. Silent mutation
D. Missense mutation
Correct answer: B. Frameshift mutations result from insertions or
deletions not in multiples of three. They alter the reading frame
downstream. This often leads to nonfunctional proteins.
14. Which hormone increases calcium reabsorption in the kidney?
A. Calcitonin
B. Parathyroid hormone
C. Aldosterone
D. Cortisol
Correct answer: B. PTH increases calcium reabsorption in the distal
tubules. It also increases phosphate excretion. This raises serum
calcium levels.
15. A patient with iron deficiency anemia is most likely to have which
lab finding?
A. Increased ferritin
B. Increased TIBC
C. Increased serum iron
D. Decreased transferrin
Correct answer: B. Iron deficiency anemia leads to increased total iron-
Questions and Answers (Accurate) Rated A+
1. A 6-month-old infant presents with poor feeding, vomiting, and
lethargy. Labs show hypoglycemia, hyperammonemia, and elevated
orotic acid. Which enzyme deficiency is most likely responsible?
A. Carbamoyl phosphate synthetase I
B. Ornithine transcarbamylase
C. Argininosuccinate synthetase
D. Arginase
Correct answer: B. Ornithine transcarbamylase deficiency leads to
excess carbamoyl phosphate, which is shunted into pyrimidine
synthesis causing elevated orotic acid. It is X-linked and commonly
presents with hyperammonemia. Hypoglycemia and vomiting are
common early signs.
2. A patient develops flushing, wheezing, and diarrhea after eating
certain foods. Which substance is most directly responsible?
A. Histamine
B. Bradykinin
C. Serotonin
D. Prostaglandin E2
Correct answer: C. Serotonin is secreted by carcinoid tumors and
causes flushing, diarrhea, and bronchospasm. It is derived from
tryptophan. Chronic excess can lead to niacin deficiency.
3. A drug that inhibits dihydrofolate reductase would most directly impair
synthesis of which molecule?
A. Thymidine
B. Cytosine
C. Guanine
D. Adenine
Correct answer: A. Dihydrofolate reductase is required for regeneration
of tetrahydrofolate, which is essential for thymidine synthesis.
Inhibition leads to impaired DNA replication. Rapidly dividing cells are
most affected.
4. A patient has loss of pain and temperature sensation on the right side
of the body and left side of the face. Where is the lesion?
A. Pons
B. Medulla
C. Midbrain
D. Cervical spinal cord
Correct answer: B. This pattern is classic for lateral medullary
(Wallenberg) syndrome. It involves damage to spinothalamic and
trigeminal pathways. The most common cause is PICA infarction.
5. A newborn has a continuous machine-like heart murmur. Which drug
promotes closure of the responsible structure?
, A. Alprostadil
B. Indomethacin
C. Nitric oxide
D. Sildenafil
Correct answer: B. Indomethacin inhibits prostaglandin synthesis,
promoting closure of the ductus arteriosus. A PDA causes a continuous
murmur. Prostaglandins normally keep the ductus open.
6. A patient with chronic kidney disease develops secondary
hyperparathyroidism. What is the primary mechanism?
A. Increased calcitriol production
B. Decreased phosphate excretion
C. Increased calcium absorption
D. Increased vitamin D activation
Correct answer: B. Reduced phosphate excretion leads to
hyperphosphatemia, which lowers serum calcium. This stimulates
parathyroid hormone release. Low calcitriol further worsens
hypocalcemia.
7. Which immunoglobulin is most important for mucosal immunity?
A. IgG
B. IgA
C. IgM
D. IgE
Correct answer: B. IgA is the predominant antibody in mucosal
secretions such as saliva and breast milk. It exists as a dimer with a
secretory component. It prevents pathogen adherence.
8. A tumor suppressor gene typically functions by which mechanism?
A. Promoting cell division
B. Activating oncogenes
C. Inhibiting cell cycle progression
D. Increasing telomerase activity
Correct answer: C. Tumor suppressor genes slow cell division or
promote apoptosis. Loss of function mutations lead to uncontrolled
growth. Both alleles usually must be inactivated.
9. A patient with Graves disease has elevated thyroid hormone levels.
Which finding is expected?
A. Increased TSH
B. Decreased T3 and T4
C. Increased radioactive iodine uptake
D. Bradycardia
Correct answer: C. Graves disease causes diffuse thyroid hyperactivity,
increasing iodine uptake. TSH is suppressed due to negative feedback.
Patients typically have tachycardia and weight loss.
10. Which vitamin deficiency causes impaired night vision?
A. Vitamin A
B. Vitamin D
C. Vitamin K
, D. Vitamin C
Correct answer: A. Vitamin A is required for rhodopsin formation in
retinal rods. Deficiency leads to night blindness. Severe cases may
cause xerophthalmia.
11. A patient presents with muscle weakness that improves with
repeated use. Which antibody is involved?
A. Acetylcholine receptor
B. Voltage-gated calcium channel
C. Sodium channel
D. Potassium channel
Correct answer: B. Lambert-Eaton syndrome involves antibodies
against presynaptic calcium channels. Repeated use increases calcium
buildup, improving strength. It is often paraneoplastic.
12. Which organism lacks a cell wall?
A. Streptococcus pneumoniae
B. Mycoplasma pneumoniae
C. Escherichia coli
D. Staphylococcus aureus
Correct answer: B. Mycoplasma lacks a peptidoglycan cell wall. This
makes beta-lactam antibiotics ineffective. It is associated with atypical
pneumonia.
13. A mutation causing a frameshift is most likely due to what
change?
A. Single nucleotide substitution
B. Deletion not divisible by three
C. Silent mutation
D. Missense mutation
Correct answer: B. Frameshift mutations result from insertions or
deletions not in multiples of three. They alter the reading frame
downstream. This often leads to nonfunctional proteins.
14. Which hormone increases calcium reabsorption in the kidney?
A. Calcitonin
B. Parathyroid hormone
C. Aldosterone
D. Cortisol
Correct answer: B. PTH increases calcium reabsorption in the distal
tubules. It also increases phosphate excretion. This raises serum
calcium levels.
15. A patient with iron deficiency anemia is most likely to have which
lab finding?
A. Increased ferritin
B. Increased TIBC
C. Increased serum iron
D. Decreased transferrin
Correct answer: B. Iron deficiency anemia leads to increased total iron-
