, Question 1
A patient with a severe penicillin allergy requires treatment for a skin infection
caused by a macrolide-resistant Streptococcus pyogenes. Which mechanism
best explains why clindamycin may still be effective despite macrolide
resistance?
A. Clindamycin binds to a different ribosomal subunit than macrolides.
B. Clindamycin is not affected by erm-mediated methylation of the 23S
rRNA.
C. Clindamycin has a higher affinity for the 50S subunit than macrolides.
D. Clindamycin is a bacteriostatic agent, whereas macrolides are
bactericidal.
Correct Answer: B - Clindamycin is not affected by erm-mediated
methylation of the 23S rRNA.
RATIONALE
Erm-mediated methylation of 23S rRNA typically confers
cross-resistance between macrolides and clindamycin (MLSb
phenotype). However, in some strains, clindamycin may retain activity
if resistance is due to an efflux pump (mef gene) that affects only
macrolides. Option B is incorrect because erm methylation does affect
clindamycin; the correct explanation is that efflux-mediated macrolide
resistance does not affect clindamycin. Option A is false (both bind
50S). Option C is not the primary reason. Option D is irrelevant to
resistance.
Question 2
Which of the following best describes the mechanism of action of linezolid?
A. Inhibition of peptide bond formation by binding to the 50S subunit's
peptidyl transferase center.
B. Blockade of the ribosomal exit tunnel, preventing elongation of the
nascent polypeptide.
C. Prevention of the initiation complex formation by binding to the 30S
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, subunit.
D. Inhibition of tRNA charging by aminoacyl-tRNA synthetases.
Correct Answer: B - Blockade of the ribosomal exit tunnel,
preventing elongation of the nascent polypeptide.
RATIONALE
Linezolid is an oxazolidinone that binds to the 50S subunit at the
peptidyl transferase center, but its unique mechanism is blocking the
ribosomal exit tunnel, preventing the nascent peptide from elongating.
Option A describes macrolides. Option C describes aminoglycosides.
Option D is not a mechanism of linezolid.
Question 3
A patient on linezolid for a resistant infection develops thrombocytopenia and
anemia. Which of the following is the most likely mechanism for these
hematologic effects?
A. Direct bone marrow suppression due to mitochondrial toxicity.
B. Immune-mediated destruction of platelets and red blood cells.
C. Inhibition of folate metabolism.
D. Increased clearance due to drug-induced liver injury.
Correct Answer: A - Direct bone marrow suppression due to
mitochondrial toxicity.
RATIONALE
Linezolid causes myelosuppression, particularly thrombocytopenia
and anemia, through reversible inhibition of mitochondrial protein
synthesis, which affects rapidly dividing hematopoietic cells. This is
not immune-mediated (B), not due to folate antagonism (C), and not
primarily via liver injury (D).
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, Question 4
A patient is prescribed clarithromycin for Helicobacter pylori eradication.
Which of the following drug interactions is most concerning due to CYP3A4
inhibition?
A. Increased metabolism of warfarin, leading to decreased INR.
B. Decreased metabolism of statins, increasing risk of rhabdomyolysis.
C. Reduced absorption of levothyroxine.
D. Enhanced renal clearance of digoxin.
Correct Answer: B - Decreased metabolism of statins, increasing
risk of rhabdomyolysis.
RATIONALE
Clarithromycin is a strong CYP3A4 inhibitor, which can significantly
increase levels of statins (e.g., simvastatin, atorvastatin), leading to
myopathy and rhabdomyolysis. Warfarin metabolism may be
inhibited, increasing INR (opposite of A). Levothyroxine absorption is
not significantly affected. Digoxin clearance may be increased by
clarithromycin? Actually clarithromycin can increase digoxin levels
by altering gut flora, not enhance renal clearance.
Question 5
Which of the following statements regarding the spectrum of activity of
azithromycin compared to erythromycin is most accurate?
A. Azithromycin has enhanced activity against Gram-negative bacilli,
including Pseudomonas aeruginosa.
B. Azithromycin has improved activity against Haemophilus influenzae
and Moraxella catarrhalis.
C. Azithromycin is less effective against Chlamydia trachomatis than
erythromycin.
D. Azithromycin lacks activity against Streptococcus pneumoniae.
Correct Answer: B - Azithromycin has improved activity against
Page 4
A patient with a severe penicillin allergy requires treatment for a skin infection
caused by a macrolide-resistant Streptococcus pyogenes. Which mechanism
best explains why clindamycin may still be effective despite macrolide
resistance?
A. Clindamycin binds to a different ribosomal subunit than macrolides.
B. Clindamycin is not affected by erm-mediated methylation of the 23S
rRNA.
C. Clindamycin has a higher affinity for the 50S subunit than macrolides.
D. Clindamycin is a bacteriostatic agent, whereas macrolides are
bactericidal.
Correct Answer: B - Clindamycin is not affected by erm-mediated
methylation of the 23S rRNA.
RATIONALE
Erm-mediated methylation of 23S rRNA typically confers
cross-resistance between macrolides and clindamycin (MLSb
phenotype). However, in some strains, clindamycin may retain activity
if resistance is due to an efflux pump (mef gene) that affects only
macrolides. Option B is incorrect because erm methylation does affect
clindamycin; the correct explanation is that efflux-mediated macrolide
resistance does not affect clindamycin. Option A is false (both bind
50S). Option C is not the primary reason. Option D is irrelevant to
resistance.
Question 2
Which of the following best describes the mechanism of action of linezolid?
A. Inhibition of peptide bond formation by binding to the 50S subunit's
peptidyl transferase center.
B. Blockade of the ribosomal exit tunnel, preventing elongation of the
nascent polypeptide.
C. Prevention of the initiation complex formation by binding to the 30S
Page 2
, subunit.
D. Inhibition of tRNA charging by aminoacyl-tRNA synthetases.
Correct Answer: B - Blockade of the ribosomal exit tunnel,
preventing elongation of the nascent polypeptide.
RATIONALE
Linezolid is an oxazolidinone that binds to the 50S subunit at the
peptidyl transferase center, but its unique mechanism is blocking the
ribosomal exit tunnel, preventing the nascent peptide from elongating.
Option A describes macrolides. Option C describes aminoglycosides.
Option D is not a mechanism of linezolid.
Question 3
A patient on linezolid for a resistant infection develops thrombocytopenia and
anemia. Which of the following is the most likely mechanism for these
hematologic effects?
A. Direct bone marrow suppression due to mitochondrial toxicity.
B. Immune-mediated destruction of platelets and red blood cells.
C. Inhibition of folate metabolism.
D. Increased clearance due to drug-induced liver injury.
Correct Answer: A - Direct bone marrow suppression due to
mitochondrial toxicity.
RATIONALE
Linezolid causes myelosuppression, particularly thrombocytopenia
and anemia, through reversible inhibition of mitochondrial protein
synthesis, which affects rapidly dividing hematopoietic cells. This is
not immune-mediated (B), not due to folate antagonism (C), and not
primarily via liver injury (D).
Page 3
, Question 4
A patient is prescribed clarithromycin for Helicobacter pylori eradication.
Which of the following drug interactions is most concerning due to CYP3A4
inhibition?
A. Increased metabolism of warfarin, leading to decreased INR.
B. Decreased metabolism of statins, increasing risk of rhabdomyolysis.
C. Reduced absorption of levothyroxine.
D. Enhanced renal clearance of digoxin.
Correct Answer: B - Decreased metabolism of statins, increasing
risk of rhabdomyolysis.
RATIONALE
Clarithromycin is a strong CYP3A4 inhibitor, which can significantly
increase levels of statins (e.g., simvastatin, atorvastatin), leading to
myopathy and rhabdomyolysis. Warfarin metabolism may be
inhibited, increasing INR (opposite of A). Levothyroxine absorption is
not significantly affected. Digoxin clearance may be increased by
clarithromycin? Actually clarithromycin can increase digoxin levels
by altering gut flora, not enhance renal clearance.
Question 5
Which of the following statements regarding the spectrum of activity of
azithromycin compared to erythromycin is most accurate?
A. Azithromycin has enhanced activity against Gram-negative bacilli,
including Pseudomonas aeruginosa.
B. Azithromycin has improved activity against Haemophilus influenzae
and Moraxella catarrhalis.
C. Azithromycin is less effective against Chlamydia trachomatis than
erythromycin.
D. Azithromycin lacks activity against Streptococcus pneumoniae.
Correct Answer: B - Azithromycin has improved activity against
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