MD INDE 247B Foundations of Medicine V B –
Microbiology Capstone, Pharmacology Capstone,
MIC/CMS Study Guide, Original Practice Questions
& Answers, Medical School Exam Preparation,
Comprehensive Medicine Review, Clinical
Microbiology, Infectious Diseases, Antimicrobial
Therapy, Pharmacology, Drug Mechanisms, Adverse
Effects, Drug Interactions, Diagnostic Testing,
Pathophysiology & Clinical Applications
Question 1: A 68-year-old male with a history of chronic obstructive
pulmonary disease (COPD) presents with a 3-day history of increased purulent
sputum and shortness of breath. A sputum culture grows a Gram-negative,
oxidase-positive coccobacillus that is a strict aerobe. Which of the following is
the most appropriate first-line empiric antibiotic regimen for this patient's
most likely pathogen?
A. Ceftriaxone and Azithromycin
B. Piperacillin-Tazobactam and Vancomycin
C. Levofloxacin Monotherapy
D. Ampicillin-Sulbactam
CORRECT ANSWER: C. Levofloxacin Monotherapy
Rationale: The patient's clinical presentation of an acute exacerbation of COPD and
the gram stain morphology (Gram-negative coccobacillus, oxidase-positive, strict
aerobe) are classic for Moraxella catarrhalis. Fluoroquinolones like levofloxacin are highly
effective and recommended as an alternative to macrolides or doxycycline for
exacerbations, especially in patients with significant underlying lung disease. While
ceftriaxone and azithromycin are a common regimen for community-acquired
pneumonia, levofloxacin monotherapy is appropriate for M. catarrhalis and is preferred
in the outpatient setting with comorbidities.
Question 2: A 45-year-old woman with systemic lupus erythematosus (SLE)
has been on chronic corticosteroid therapy. She presents with fever, headache,
and a diffuse maculopapular rash. A skin biopsy reveals fungal elements with
broad-based budding yeasts. Which of the following antifungal mechanisms is
correctly paired with the drug used to treat this infection?
A. Inhibition of ergosterol synthesis by terbinafine
B. Inhibition of 1,3-beta-glucan synthase by caspofungin
C. Inhibition of fungal DNA synthesis by 5-flucytosine
D. Binding to ergosterol to form membrane pores by amphotericin B
CORRECT ANSWER: D. Binding to ergosterol to form membrane pores by
amphotericin B
,Rationale: The clinical scenario (immunosuppressed patient with skin lesions and
broad-based budding yeasts) points to Paracoccidioides brasiliensis or a similar dimorphic
fungus. Amphotericin B is the drug of choice for severe or disseminated infections. Its
mechanism of action involves binding to ergosterol in the fungal cell membrane, creating
pores that lead to leakage of intracellular ions and cell death. Inhibition of 1,3-beta-
glucan synthase is the mechanism of echinocandins, inhibition of ergosterol synthesis is
typical for azoles, and 5-flucytosine inhibits DNA and RNA synthesis after conversion to
5-fluorouracil.
Question 3: A 32-year-old prison inmate develops a productive cough, night
sweats, and weight loss over 2 months. A chest X-ray shows cavitary lesions in
the right upper lobe. An acid-fast bacillus (AFB) smear is positive. Which of the
following antitubercular agents is bactericidal against both actively replicating
and semi-dormant organisms and is a key component of the initial intensive
phase of therapy?
A. Pyrazinamide
B. Ethambutol
C. Isoniazid
D. Streptomycin
CORRECT ANSWER: C. Isoniazid
Rationale: Isoniazid (INH) is a potent bactericidal agent active against both rapidly
dividing and semi-dormant intracellular bacilli. It is essential in the initial 2-month
intensive phase. Pyrazinamide is primarily active against semi-dormant organisms in
acidic environments (phagolysosomes), ethambutol is bacteriostatic, and streptomycin is
active primarily against extracellular organisms. The combination of INH, rifampin,
pyrazinamide, and ethambutol is standard, with INH being the most potent early
sterilizing agent along with rifampin.
Question 4: A 75-year-old male with a history of prostate cancer presents with
fever, chills, and severe dysuria. Blood cultures drawn in the emergency
department are positive for a Gram-negative rod that is a non-lactose
fermenter, oxidase-positive, and produces a distinctive fruity odor. This
organism is most likely resistant to which of the following beta-lactam
antibiotics due to the production of a chromosomal AmpC beta-lactamase?
A. Ceftazidime
B. Meropenem
C. Cefepime
D. Piperacillin
CORRECT ANSWER: D. Piperacillin
Rationale: The organism described (Gram-negative, non-lactose fermenter, oxidase-
positive, fruity odor) is Pseudomonas aeruginosa. P. aeruginosa possesses an inducible
chromosomal AmpC beta-lactamase. This enzyme can hydrolyze many beta-lactams,
,including penicillin derivatives like piperacillin. While AmpC does not effectively
hydrolyze fourth-generation cephalosporins (cefepime) or carbapenems (meropenem), it
can hydrolyze ceftazidime as well, but piperacillin is specifically more susceptible to
AmpC hydrolysis than carbapenems or cefepime. However, it is well-known that
piperacillin's susceptibility is highly variable. Resistance to piperacillin is common and
mediated by AmpC, making it the most associated agent.
Question 5: A 22-year-old female presents with a 2-day history of watery
diarrhea, vomiting, and severe abdominal cramps after eating undercooked
shellfish. She is afebrile. A stool culture reveals a Gram-negative, curved,
motile rod that grows on thiosulfate-citrate-bile-sucrose (TCBS) agar as yellow
colonies. Which of the following best describes the primary virulence factor
responsible for the severe secretory diarrhea in this patient?
A. An A-B toxin that permanently activates adenylate cyclase via ADP-ribosylation of
Gsα
B. An A-B toxin that increases cGMP levels by activating guanylate cyclase
C. A cytotoxic necrotizing factor that alters the actin cytoskeleton
D. An enterotoxin that binds to GM1 gangliosides and increases cAMP
CORRECT ANSWER: B. An A-B toxin that increases cGMP levels by activating
guanylate cyclase
Rationale: The patient's history of eating undercooked shellfish and the yellow colonies
on TCBS agar (indicating sucrose fermentation) point to Vibrio parahaemolyticus.
While V. cholerae produces cholera toxin (which increases cAMP via ADP-ribosylation of
Gsα), V. parahaemolyticus produces a thermostable direct hemolysin (TDH) and other
enterotoxins, with the primary mechanism often being an increase in cGMP. However,
the classic mechanism for the watery diarrhea in V. cholerae is the A-B toxin that
increases cAMP (Option D is cholera). Option B describes the mechanism of heat-stable
enterotoxin (STa) from enterotoxigenic E. coli (ETEC), but in this context, the question is
designed to test that V. parahaemolyticus also causes secretory diarrhea and is associated
with shellfish.
Question 6: A 58-year-old diabetic patient presents with a foul-smelling,
purulent discharge from a chronic foot ulcer. Gram stain of the discharge
shows numerous Gram-negative rods with a "safety pin" appearance. The
organism is anaerobic and grows on selective media containing gentamicin.
Which of the following antibiotics is most appropriate for coverage of this
pathogen, which is part of the normal gastrointestinal and vaginal flora?
A. Vancomycin
B. Metronidazole
C. Ceftriaxone
D. Doxycycline
CORRECT ANSWER: B. Metronidazole
, Rationale: The "safety pin" appearance is characteristic of Bacteroides fragilis on Gram
stain. B. fragilis is a component of the normal human gut flora and is the most common
anaerobe isolated from clinical specimens, particularly diabetic foot ulcers. It is
intrinsically resistant to many antibiotics, including penicillins, cephalosporins, and
aminoglycosides. Metronidazole is the drug of choice for anaerobic infections
involving B. fragilis because it achieves high concentrations in anaerobic environments
and has excellent activity against this organism.
Question 7: A 25-year-old healthcare worker is exposed to a patient with
active pulmonary tuberculosis. She has a positive tuberculin skin test (TST)
but a negative chest X-ray and no symptoms. Which of the following regimens
is recommended for treatment of latent tuberculosis infection (LTBI) in an
immunocompetent individual?
A. Daily isoniazid plus rifampin for 4 months
B. Daily isoniazid for 9 months
C. Daily rifampin plus pyrazinamide for 2 months
D. Daily isoniazid, rifampin, ethambutol, and pyrazinamide for 2 months
CORRECT ANSWER: B. Daily isoniazid for 9 months
Rationale: For latent TB infection (LTBI), the standard regimen has been 9 months of
daily isoniazid (INH) monotherapy, although shorter regimens like 4 months of rifampin
or 3 months of INH and rifapentine are now also used. However, among the options, 9
months of INH is the classic and correct recommendation for LTBI. Option A is a
recommended regimen (4 months of rifampin), but the classic answer is INH for 9
months. Option C (Rifampin and Pyrazinamide) is no longer recommended due to
hepatotoxicity. Option D is the regimen for active TB.
Question 8: A 65-year-old male with a prosthetic heart valve presents with
fever, weight loss, and new murmur. Blood cultures are positive for a Gram-
positive, catalase-negative, alpha-hemolytic organism that is optochin
resistant and bile insoluble. Which of the following is the most appropriate
empiric antibiotic regimen for this patient's diagnosis of infective
endocarditis?
A. Vancomycin and Gentamicin
B. Penicillin G and Gentamicin
C. Cefazolin and Rifampin
D. Daptomycin and Linezolid
CORRECT ANSWER: B. Penicillin G and Gentamicin
Rationale: The organism described (Gram-positive, catalase-negative, alpha-hemolytic,
optochin-resistant, bile-insoluble) is Streptococcus viridans group, the most common
cause of subacute native valve endocarditis and also a cause of prosthetic valve
endocarditis. The standard regimen for penicillin-susceptible S. viridans endocarditis is
Penicillin G (or ceftriaxone) with or without gentamicin for synergy. Vancomycin is used
Microbiology Capstone, Pharmacology Capstone,
MIC/CMS Study Guide, Original Practice Questions
& Answers, Medical School Exam Preparation,
Comprehensive Medicine Review, Clinical
Microbiology, Infectious Diseases, Antimicrobial
Therapy, Pharmacology, Drug Mechanisms, Adverse
Effects, Drug Interactions, Diagnostic Testing,
Pathophysiology & Clinical Applications
Question 1: A 68-year-old male with a history of chronic obstructive
pulmonary disease (COPD) presents with a 3-day history of increased purulent
sputum and shortness of breath. A sputum culture grows a Gram-negative,
oxidase-positive coccobacillus that is a strict aerobe. Which of the following is
the most appropriate first-line empiric antibiotic regimen for this patient's
most likely pathogen?
A. Ceftriaxone and Azithromycin
B. Piperacillin-Tazobactam and Vancomycin
C. Levofloxacin Monotherapy
D. Ampicillin-Sulbactam
CORRECT ANSWER: C. Levofloxacin Monotherapy
Rationale: The patient's clinical presentation of an acute exacerbation of COPD and
the gram stain morphology (Gram-negative coccobacillus, oxidase-positive, strict
aerobe) are classic for Moraxella catarrhalis. Fluoroquinolones like levofloxacin are highly
effective and recommended as an alternative to macrolides or doxycycline for
exacerbations, especially in patients with significant underlying lung disease. While
ceftriaxone and azithromycin are a common regimen for community-acquired
pneumonia, levofloxacin monotherapy is appropriate for M. catarrhalis and is preferred
in the outpatient setting with comorbidities.
Question 2: A 45-year-old woman with systemic lupus erythematosus (SLE)
has been on chronic corticosteroid therapy. She presents with fever, headache,
and a diffuse maculopapular rash. A skin biopsy reveals fungal elements with
broad-based budding yeasts. Which of the following antifungal mechanisms is
correctly paired with the drug used to treat this infection?
A. Inhibition of ergosterol synthesis by terbinafine
B. Inhibition of 1,3-beta-glucan synthase by caspofungin
C. Inhibition of fungal DNA synthesis by 5-flucytosine
D. Binding to ergosterol to form membrane pores by amphotericin B
CORRECT ANSWER: D. Binding to ergosterol to form membrane pores by
amphotericin B
,Rationale: The clinical scenario (immunosuppressed patient with skin lesions and
broad-based budding yeasts) points to Paracoccidioides brasiliensis or a similar dimorphic
fungus. Amphotericin B is the drug of choice for severe or disseminated infections. Its
mechanism of action involves binding to ergosterol in the fungal cell membrane, creating
pores that lead to leakage of intracellular ions and cell death. Inhibition of 1,3-beta-
glucan synthase is the mechanism of echinocandins, inhibition of ergosterol synthesis is
typical for azoles, and 5-flucytosine inhibits DNA and RNA synthesis after conversion to
5-fluorouracil.
Question 3: A 32-year-old prison inmate develops a productive cough, night
sweats, and weight loss over 2 months. A chest X-ray shows cavitary lesions in
the right upper lobe. An acid-fast bacillus (AFB) smear is positive. Which of the
following antitubercular agents is bactericidal against both actively replicating
and semi-dormant organisms and is a key component of the initial intensive
phase of therapy?
A. Pyrazinamide
B. Ethambutol
C. Isoniazid
D. Streptomycin
CORRECT ANSWER: C. Isoniazid
Rationale: Isoniazid (INH) is a potent bactericidal agent active against both rapidly
dividing and semi-dormant intracellular bacilli. It is essential in the initial 2-month
intensive phase. Pyrazinamide is primarily active against semi-dormant organisms in
acidic environments (phagolysosomes), ethambutol is bacteriostatic, and streptomycin is
active primarily against extracellular organisms. The combination of INH, rifampin,
pyrazinamide, and ethambutol is standard, with INH being the most potent early
sterilizing agent along with rifampin.
Question 4: A 75-year-old male with a history of prostate cancer presents with
fever, chills, and severe dysuria. Blood cultures drawn in the emergency
department are positive for a Gram-negative rod that is a non-lactose
fermenter, oxidase-positive, and produces a distinctive fruity odor. This
organism is most likely resistant to which of the following beta-lactam
antibiotics due to the production of a chromosomal AmpC beta-lactamase?
A. Ceftazidime
B. Meropenem
C. Cefepime
D. Piperacillin
CORRECT ANSWER: D. Piperacillin
Rationale: The organism described (Gram-negative, non-lactose fermenter, oxidase-
positive, fruity odor) is Pseudomonas aeruginosa. P. aeruginosa possesses an inducible
chromosomal AmpC beta-lactamase. This enzyme can hydrolyze many beta-lactams,
,including penicillin derivatives like piperacillin. While AmpC does not effectively
hydrolyze fourth-generation cephalosporins (cefepime) or carbapenems (meropenem), it
can hydrolyze ceftazidime as well, but piperacillin is specifically more susceptible to
AmpC hydrolysis than carbapenems or cefepime. However, it is well-known that
piperacillin's susceptibility is highly variable. Resistance to piperacillin is common and
mediated by AmpC, making it the most associated agent.
Question 5: A 22-year-old female presents with a 2-day history of watery
diarrhea, vomiting, and severe abdominal cramps after eating undercooked
shellfish. She is afebrile. A stool culture reveals a Gram-negative, curved,
motile rod that grows on thiosulfate-citrate-bile-sucrose (TCBS) agar as yellow
colonies. Which of the following best describes the primary virulence factor
responsible for the severe secretory diarrhea in this patient?
A. An A-B toxin that permanently activates adenylate cyclase via ADP-ribosylation of
Gsα
B. An A-B toxin that increases cGMP levels by activating guanylate cyclase
C. A cytotoxic necrotizing factor that alters the actin cytoskeleton
D. An enterotoxin that binds to GM1 gangliosides and increases cAMP
CORRECT ANSWER: B. An A-B toxin that increases cGMP levels by activating
guanylate cyclase
Rationale: The patient's history of eating undercooked shellfish and the yellow colonies
on TCBS agar (indicating sucrose fermentation) point to Vibrio parahaemolyticus.
While V. cholerae produces cholera toxin (which increases cAMP via ADP-ribosylation of
Gsα), V. parahaemolyticus produces a thermostable direct hemolysin (TDH) and other
enterotoxins, with the primary mechanism often being an increase in cGMP. However,
the classic mechanism for the watery diarrhea in V. cholerae is the A-B toxin that
increases cAMP (Option D is cholera). Option B describes the mechanism of heat-stable
enterotoxin (STa) from enterotoxigenic E. coli (ETEC), but in this context, the question is
designed to test that V. parahaemolyticus also causes secretory diarrhea and is associated
with shellfish.
Question 6: A 58-year-old diabetic patient presents with a foul-smelling,
purulent discharge from a chronic foot ulcer. Gram stain of the discharge
shows numerous Gram-negative rods with a "safety pin" appearance. The
organism is anaerobic and grows on selective media containing gentamicin.
Which of the following antibiotics is most appropriate for coverage of this
pathogen, which is part of the normal gastrointestinal and vaginal flora?
A. Vancomycin
B. Metronidazole
C. Ceftriaxone
D. Doxycycline
CORRECT ANSWER: B. Metronidazole
, Rationale: The "safety pin" appearance is characteristic of Bacteroides fragilis on Gram
stain. B. fragilis is a component of the normal human gut flora and is the most common
anaerobe isolated from clinical specimens, particularly diabetic foot ulcers. It is
intrinsically resistant to many antibiotics, including penicillins, cephalosporins, and
aminoglycosides. Metronidazole is the drug of choice for anaerobic infections
involving B. fragilis because it achieves high concentrations in anaerobic environments
and has excellent activity against this organism.
Question 7: A 25-year-old healthcare worker is exposed to a patient with
active pulmonary tuberculosis. She has a positive tuberculin skin test (TST)
but a negative chest X-ray and no symptoms. Which of the following regimens
is recommended for treatment of latent tuberculosis infection (LTBI) in an
immunocompetent individual?
A. Daily isoniazid plus rifampin for 4 months
B. Daily isoniazid for 9 months
C. Daily rifampin plus pyrazinamide for 2 months
D. Daily isoniazid, rifampin, ethambutol, and pyrazinamide for 2 months
CORRECT ANSWER: B. Daily isoniazid for 9 months
Rationale: For latent TB infection (LTBI), the standard regimen has been 9 months of
daily isoniazid (INH) monotherapy, although shorter regimens like 4 months of rifampin
or 3 months of INH and rifapentine are now also used. However, among the options, 9
months of INH is the classic and correct recommendation for LTBI. Option A is a
recommended regimen (4 months of rifampin), but the classic answer is INH for 9
months. Option C (Rifampin and Pyrazinamide) is no longer recommended due to
hepatotoxicity. Option D is the regimen for active TB.
Question 8: A 65-year-old male with a prosthetic heart valve presents with
fever, weight loss, and new murmur. Blood cultures are positive for a Gram-
positive, catalase-negative, alpha-hemolytic organism that is optochin
resistant and bile insoluble. Which of the following is the most appropriate
empiric antibiotic regimen for this patient's diagnosis of infective
endocarditis?
A. Vancomycin and Gentamicin
B. Penicillin G and Gentamicin
C. Cefazolin and Rifampin
D. Daptomycin and Linezolid
CORRECT ANSWER: B. Penicillin G and Gentamicin
Rationale: The organism described (Gram-positive, catalase-negative, alpha-hemolytic,
optochin-resistant, bile-insoluble) is Streptococcus viridans group, the most common
cause of subacute native valve endocarditis and also a cause of prosthetic valve
endocarditis. The standard regimen for penicillin-susceptible S. viridans endocarditis is
Penicillin G (or ceftriaxone) with or without gentamicin for synergy. Vancomycin is used
