, Question 1
A patient with a documented QT interval of 510 ms and a history of torsades
de pointes is prescribed a fluoroquinolone for a complicated urinary tract
infection. Which agent poses the greatest additional risk of QT prolongation
and should be avoided?
A. Ciprofloxacin
B. Levofloxacin
C. Moxifloxacin
D. Ofloxacin
Correct Answer: C - Moxifloxacin
RATIONALE
Moxifloxacin carries the strongest QT-prolongation signal among
fluoroquinolones and is contraindicated in patients with baseline QT
prolongation or a history of torsades. Levofloxacin and ciprofloxacin
have moderate risk, while ofloxacin has comparatively lower risk;
however, all fluoroquinolones should be used cautiously in this
scenario.
Question 2
Which statement best explains why trimethoprim is often combined with
sulfamethoxazole in clinical practice?
A. The combination expands coverage to Pseudomonas aeruginosa.
B. Sequential blockade of the folate synthesis pathway produces
synergistic bactericidal activity.
C. Sulfamethoxazole prevents the renal excretion of trimethoprim.
D. Trimethoprim inactivates beta-lactamases produced by resistant
organisms.
Correct Answer: B - Sequential blockade of the folate synthesis
pathway produces synergistic bactericidal activity.
Page 2
, RATIONALE
Trimethoprim inhibits dihydrofolate reductase, while
sulfamethoxazole inhibits dihydropteroate synthase, blocking two
consecutive steps in bacterial folate synthesis and producing
synergistic bacteriostatic to bactericidal effects. The combination does
not extend coverage to Pseudomonas, does not affect renal excretion,
and has no beta-lactamase inhibitory activity.
Question 3
A patient on warfarin is prescribed nitrofurantoin for an uncomplicated lower
urinary tract infection. Which monitoring parameter is most critical to assess
within the first week of combined therapy?
A. Serum creatinine and BUN
B. International normalized ratio (INR)
C. Complete blood count with differential
D. Liver function tests
Correct Answer: B - International normalized ratio (INR)
RATIONALE
Nitrofurantoin can potentiate the anticoagulant effect of warfarin,
likely through inhibition of CYP2C9-mediated metabolism or
alterations in gut flora producing vitamin K, necessitating close INR
monitoring. While renal and hematologic parameters are relevant for
nitrofurantoin toxicity, the most immediate and dangerous interaction
is the elevated bleeding risk reflected by INR.
Question 4
Which fluoroquinolone characteristic most directly accounts for its efficacy
against intracellular pathogens such as Legionella pneumophila?
A. High oral bioavailability
B. Concentration-dependent bactericidal activity
C. Excellent penetration into alveolar macrophages and phagocytes
Page 3
, D. Inhibition of topoisomerase IV rather than DNA gyrase
Correct Answer: C - Excellent penetration into alveolar
macrophages and phagocytes
RATIONALE
Fluoroquinolones achieve high intracellular concentrations within
macrophages and neutrophils, which is essential for eradicating
facultative intracellular organisms like Legionella. While
bioavailability and concentration-dependent killing contribute to
efficacy, the ability to penetrate phagocytes is the most direct
determinant of activity against intracellular pathogens.
Question 5
A patient with G6PD deficiency is prescribed dapsone for dermatitis
herpetiformis. Which adverse effect is most directly attributable to the patient's
enzyme deficiency?
A. Methemoglobinemia and hemolytic anemia
B. Agranulocytosis
C. Aplastic anemia
D. Megaloblastic anemia
Correct Answer: A - Methemoglobinemia and hemolytic anemia
RATIONALE
Dapsone, like other oxidant drugs, can precipitate methemoglobinemia
and hemolytic anemia in G6PD-deficient individuals due to impaired
regeneration of NADPH and reduced glutathione. Agranulocytosis and
aplastic anemia are idiosyncratic reactions not directly linked to G6PD
status, and megaloblastic anemia is typically associated with folate or
B12 deficiency.
Question 6
Which of the following best describes the mechanism by which methenamine
hippurate exerts its antibacterial effect in the urinary tract?
Page 4
A patient with a documented QT interval of 510 ms and a history of torsades
de pointes is prescribed a fluoroquinolone for a complicated urinary tract
infection. Which agent poses the greatest additional risk of QT prolongation
and should be avoided?
A. Ciprofloxacin
B. Levofloxacin
C. Moxifloxacin
D. Ofloxacin
Correct Answer: C - Moxifloxacin
RATIONALE
Moxifloxacin carries the strongest QT-prolongation signal among
fluoroquinolones and is contraindicated in patients with baseline QT
prolongation or a history of torsades. Levofloxacin and ciprofloxacin
have moderate risk, while ofloxacin has comparatively lower risk;
however, all fluoroquinolones should be used cautiously in this
scenario.
Question 2
Which statement best explains why trimethoprim is often combined with
sulfamethoxazole in clinical practice?
A. The combination expands coverage to Pseudomonas aeruginosa.
B. Sequential blockade of the folate synthesis pathway produces
synergistic bactericidal activity.
C. Sulfamethoxazole prevents the renal excretion of trimethoprim.
D. Trimethoprim inactivates beta-lactamases produced by resistant
organisms.
Correct Answer: B - Sequential blockade of the folate synthesis
pathway produces synergistic bactericidal activity.
Page 2
, RATIONALE
Trimethoprim inhibits dihydrofolate reductase, while
sulfamethoxazole inhibits dihydropteroate synthase, blocking two
consecutive steps in bacterial folate synthesis and producing
synergistic bacteriostatic to bactericidal effects. The combination does
not extend coverage to Pseudomonas, does not affect renal excretion,
and has no beta-lactamase inhibitory activity.
Question 3
A patient on warfarin is prescribed nitrofurantoin for an uncomplicated lower
urinary tract infection. Which monitoring parameter is most critical to assess
within the first week of combined therapy?
A. Serum creatinine and BUN
B. International normalized ratio (INR)
C. Complete blood count with differential
D. Liver function tests
Correct Answer: B - International normalized ratio (INR)
RATIONALE
Nitrofurantoin can potentiate the anticoagulant effect of warfarin,
likely through inhibition of CYP2C9-mediated metabolism or
alterations in gut flora producing vitamin K, necessitating close INR
monitoring. While renal and hematologic parameters are relevant for
nitrofurantoin toxicity, the most immediate and dangerous interaction
is the elevated bleeding risk reflected by INR.
Question 4
Which fluoroquinolone characteristic most directly accounts for its efficacy
against intracellular pathogens such as Legionella pneumophila?
A. High oral bioavailability
B. Concentration-dependent bactericidal activity
C. Excellent penetration into alveolar macrophages and phagocytes
Page 3
, D. Inhibition of topoisomerase IV rather than DNA gyrase
Correct Answer: C - Excellent penetration into alveolar
macrophages and phagocytes
RATIONALE
Fluoroquinolones achieve high intracellular concentrations within
macrophages and neutrophils, which is essential for eradicating
facultative intracellular organisms like Legionella. While
bioavailability and concentration-dependent killing contribute to
efficacy, the ability to penetrate phagocytes is the most direct
determinant of activity against intracellular pathogens.
Question 5
A patient with G6PD deficiency is prescribed dapsone for dermatitis
herpetiformis. Which adverse effect is most directly attributable to the patient's
enzyme deficiency?
A. Methemoglobinemia and hemolytic anemia
B. Agranulocytosis
C. Aplastic anemia
D. Megaloblastic anemia
Correct Answer: A - Methemoglobinemia and hemolytic anemia
RATIONALE
Dapsone, like other oxidant drugs, can precipitate methemoglobinemia
and hemolytic anemia in G6PD-deficient individuals due to impaired
regeneration of NADPH and reduced glutathione. Agranulocytosis and
aplastic anemia are idiosyncratic reactions not directly linked to G6PD
status, and megaloblastic anemia is typically associated with folate or
B12 deficiency.
Question 6
Which of the following best describes the mechanism by which methenamine
hippurate exerts its antibacterial effect in the urinary tract?
Page 4
