A 26-year-old female presents to her primary care physician concerned that she has contracted a sexually transmitted disease. She states that she is having severe pain whenever she urinates and seems to be urinating more frequently than normal. She reports that her symptoms started after she began having unprotected sexual intercourse with 1 partner earlier this week. The physician obtains a urinalysis which demonstrates the following, SG: 1.010, Leukocyte esterase: Positive, Nitrites: Positive, Protein: Trace, pH: 5.0, RBC: Negative. A urease test is performed which is negative. This patient has most likely been infected with which of the following organisms?
Q62
A 45-year-old male presents to his primary care physician complaining of drainage from his left great toe. He has had an ulcer on his left great toe for over eight months. He noticed increasing drainage from the ulcer over the past week. His past medical history is notable for diabetes mellitus on insulin complicated by peripheral neuropathy and retinopathy. His most recent hemoglobin A1c was 9.4%. He has a 25 pack-year smoking history. He has multiple sexual partners and does not use condoms. His temperature is 100.8°F (38.2°C), blood pressure is 150/70 mmHg, pulse is 100/min, and respirations are 18/min. Physical examination reveals a 1 cm ulcer on the plantar aspect of the left great toe surrounded by an edematous and erythematous ring. Exposed bone can be palpated with a probe. There are multiple small cuts and bruises on both feet. A bone biopsy reveals abundant gram-negative rods that do not ferment lactose. The pathogen most likely responsible for this patient’s current condition is also strongly associated with which of the following conditions?
Q63
Blood cultures are sent to the laboratory and empiric treatment with intravenous vancomycin is started. Blood cultures grow gram-negative bacilli identified as Cardiobacterium hominis. Which of the following is the most appropriate next step in management?
Q64
A 26-year-old woman presents to the emergency department with fever, chills, lower quadrant abdominal pain, and urinary frequency for the past week. Her vital signs include temperature 38.9°C (102.0°F), pulse 110/min, respirations 16/min, and blood pressure 122/78 mm Hg. Physical examination is unremarkable. Urinalysis reveals polymorphonuclear leukocytes (PMNs) > 10 cells/HPF and the presence of bacteria (> 105 CFU/mL). Which of the following is correct concerning the most likely microorganism responsible for this patient’s condition?
Q65
A 51-year-old man comes to the physician because of a 4-day history of fever and cough productive of foul-smelling, dark red, gelatinous sputum. He has smoked 1 pack of cigarettes daily for 30 years and drinks two 12-oz bottles of beer daily. An x-ray of the chest shows a cavity with air-fluid levels in the right lower lobe. Sputum culture grows gram-negative rods. Which of the following virulence factors is most likely involved in the pathogenesis of this patient's condition?
Q66
An 18-year-old female returning from a trip to a developing country presents with diarrhea and pain in the abdominal region. Microscopic evaluation of the stool reveals the presence of RBC's and WBC's. The patient reports poor sewage sanitation in the region she visited. The physician suspects a bacterial infection and culture reveals Gram-negative rods that are non-lactose fermenting. The A subunit of the bacteria's toxin acts to:
Gram-negative US Medical PG Practice Questions and MCQs
Question 61: A 26-year-old female presents to her primary care physician concerned that she has contracted a sexually transmitted disease. She states that she is having severe pain whenever she urinates and seems to be urinating more frequently than normal. She reports that her symptoms started after she began having unprotected sexual intercourse with 1 partner earlier this week. The physician obtains a urinalysis which demonstrates the following, SG: 1.010, Leukocyte esterase: Positive, Nitrites: Positive, Protein: Trace, pH: 5.0, RBC: Negative. A urease test is performed which is negative. This patient has most likely been infected with which of the following organisms?
A. Enterobacter cloacae
B. Staphylococcus saprophyticus
C. Proteus mirabilis
D. Klebsiella pneumoniae
E. Escherichia coli (Correct Answer)
Explanation: ***Escherichia coli***
- The urinalysis findings of **positive leukocyte esterase**, **nitrites**, and **trace protein** with a slightly acidic pH (5.0) are highly suggestive of a **urinary tract infection (UTI)**.
- *E. coli* is the most common cause of UTIs, especially in young, sexually active women, and is typically **urease-negative**, consistent with the information provided.
- *E. coli* accounts for **80-90% of uncomplicated UTIs** and produces nitrites from dietary nitrates, making it the most likely pathogen in this clinical scenario.
*Enterobacter cloacae*
- While *Enterobacter cloacae* can cause UTIs, it is less common than *E. coli* in uncomplicated cases and is often associated with nosocomial infections or those in immunocompromised individuals.
- Its urease activity can vary, so a negative urease test doesn't rule it out completely but makes *E. coli* a more likely primary choice in this context.
*Staphylococcus saprophyticus*
- *S. saprophyticus* is a common cause of UTIs in young, sexually active women (second most common cause after *E. coli*) and is typically **urease-negative**, which is consistent with the negative test.
- However, the presence of **positive nitrites** points more strongly towards **Gram-negative bacteria** like *E. coli*, as *S. saprophyticus* is a **Gram-positive coccus** that does not produce nitrite reductase and therefore does not convert nitrates to nitrites.
*Proteus mirabilis*
- *Proteus mirabilis* is known for causing UTIs and is characteristically **urease-positive**, leading to alkaline urine (higher pH) and sometimes **struvite stones**.
- The **negative urease test** and acidic urine pH (5.0) in this case effectively rule out *Proteus mirabilis*.
*Klebsiella pneumoniae*
- *Klebsiella pneumoniae* can cause UTIs and is generally **urease-negative**, but it is less frequently the cause of uncomplicated UTIs compared to *E. coli*.
- Although it can produce nitrites, *E. coli* remains the most common etiology in this clinical scenario.
Question 62: A 45-year-old male presents to his primary care physician complaining of drainage from his left great toe. He has had an ulcer on his left great toe for over eight months. He noticed increasing drainage from the ulcer over the past week. His past medical history is notable for diabetes mellitus on insulin complicated by peripheral neuropathy and retinopathy. His most recent hemoglobin A1c was 9.4%. He has a 25 pack-year smoking history. He has multiple sexual partners and does not use condoms. His temperature is 100.8°F (38.2°C), blood pressure is 150/70 mmHg, pulse is 100/min, and respirations are 18/min. Physical examination reveals a 1 cm ulcer on the plantar aspect of the left great toe surrounded by an edematous and erythematous ring. Exposed bone can be palpated with a probe. There are multiple small cuts and bruises on both feet. A bone biopsy reveals abundant gram-negative rods that do not ferment lactose. The pathogen most likely responsible for this patient’s current condition is also strongly associated with which of the following conditions?
A. Otitis externa (Correct Answer)
B. Waterhouse-Friedrichsen syndrome
C. Gastroenteritis
D. Toxic shock syndrome
E. Rheumatic fever
Explanation: ***Otitis externa***
- The patient's presentation with a chronic **diabetic foot ulcer** with exposed bone and **gram-negative, non-lactose fermenting rods** on bone biopsy indicates **osteomyelitis** caused by ***Pseudomonas aeruginosa***.
- ***Pseudomonas aeruginosa*** is strongly associated with **otitis externa** (swimmer's ear), particularly **malignant otitis externa** in diabetic and immunocompromised patients.
- This is a classic association tested on USMLE: *Pseudomonas* causes both diabetic foot osteomyelitis and otitis externa.
*Waterhouse-Friedrichsen syndrome*
- This syndrome involves adrenal hemorrhage and fulminant sepsis, classically caused by ***Neisseria meningitidis***.
- Not associated with *Pseudomonas aeruginosa*.
*Gastroenteritis*
- Primarily caused by enteric pathogens such as *Salmonella*, *Shigella*, *Campylobacter*, *E. coli*, or viral agents.
- *Pseudomonas aeruginosa* is not a typical cause of gastroenteritis.
*Toxic shock syndrome*
- Caused by exotoxins from ***Staphylococcus aureus*** (TSST-1) or **Group A Streptococcus** (pyrogenic exotoxins).
- Not associated with *Pseudomonas aeruginosa*.
*Rheumatic fever*
- A delayed autoimmune complication of **Group A Streptococcal pharyngitis**.
- Not related to *Pseudomonas* infections or diabetic foot ulcers.
Question 63: Blood cultures are sent to the laboratory and empiric treatment with intravenous vancomycin is started. Blood cultures grow gram-negative bacilli identified as Cardiobacterium hominis. Which of the following is the most appropriate next step in management?
A. Switch to intravenous gentamicin
B. Switch to intravenous ampicillin
C. Switch to intravenous ceftriaxone (Correct Answer)
D. Switch to intravenous cefazolin
E. Add intravenous rifampin
Explanation: ***Switch to intravenous ceftriaxone***
- **Cardiobacterium hominis** is part of the **HACEK group** of bacteria, which are known for causing **endocarditis**.
- These organisms are typically susceptible to **beta-lactam antibiotics**, with **third-generation cephalosporins** like ceftriaxone being the drug of choice due to their excellent activity and good penetration.
*Switch to intravenous gentamicin*
- While **aminoglycosides** like gentamicin can be used in combination regimens for serious infections, they are generally **not monotherapy** for HACEK endocarditis and are associated with **nephrotoxicity** and **ototoxicity**.
- The primary treatment for HACEK endocarditis is a **beta-lactam antibiotic**, not an aminoglycoside alone.
*Switch to intravenous ampicillin*
- **Ampicillin** is a beta-lactam, but it may not consistently provide optimal coverage for all HACEK organisms, and some strains may have reduced susceptibility.
- **Third-generation cephalosporins** are preferred due to their broader and more consistent activity against this group.
*Switch to intravenous cefazolin*
- **Cefazolin** is a first-generation cephalosporin and typically has **limited activity** against gram-negative bacilli, especially those like Cardiobacterium hominis which require broader-spectrum beta-lactams.
- Its spectrum of activity is primarily against **gram-positive bacteria** and some **gram-negative cocci**.
*Add intravenous rifampin*
- **Rifampin** is primarily used for **mycobacterial infections** and in combination regimens for specific bacterial infections (e.g., bone and joint infections, prosthetic device infections) often due to resistant staphylococci.
- It is **not a first-line agent** for Cardiobacterium hominis infections and there's no indication for its use here with an organism susceptible to ceftriaxone.
Question 64: A 26-year-old woman presents to the emergency department with fever, chills, lower quadrant abdominal pain, and urinary frequency for the past week. Her vital signs include temperature 38.9°C (102.0°F), pulse 110/min, respirations 16/min, and blood pressure 122/78 mm Hg. Physical examination is unremarkable. Urinalysis reveals polymorphonuclear leukocytes (PMNs) > 10 cells/HPF and the presence of bacteria (> 105 CFU/mL). Which of the following is correct concerning the most likely microorganism responsible for this patient’s condition?
A. Gram-positive cocci that grow in clusters
B. Pear-shaped motile protozoa
C. Gram-positive cocci that grow in chains
D. Gram-negative rod-shaped bacilli (Correct Answer)
E. Nonmotile, pleomorphic rod-shaped, gram-negative bacilli
Explanation: ***Gram-negative rod-shaped bacilli***
- The symptoms of **fever, chills, abdominal pain, and urinary frequency**, along with urinalysis showing **pyuria (>10 PMNs/HPF) and bacteriuria (>10^5 CFU/mL)**, are highly suggestive of a **urinary tract infection (UTI)**.
- **_Escherichia coli_**, a **gram-negative rod-shaped bacillus**, is the most common cause of community-acquired UTIs, accounting for over 80% of cases.
*Gram-positive cocci that grow in clusters*
- This morphology describes **Staphylococcus species**, which can cause UTIs but are **less common** than *E. coli* in uncomplicated cases.
- **_Staphylococcus saprophyticus_** is a notable cause of UTIs in young, sexually active women, but it is not the most likely overall.
*Pear-shaped motile protozoa*
- This description typically refers to **_Giardia lamblia_**, which causes **gastrointestinal infections** (giardiasis) and is not associated with UTIs.
- UTIs are bacterial infections, not protozoal.
*Gram-positive cocci that grow in chains*
- This morphology describes **Streptococcus species**, such as **_Streptococcus agalactiae_** (Group B Streptococcus), which can cause UTIs, especially during pregnancy.
- However, they are **less frequent causes** of uncomplicated UTIs compared to *E. coli*.
*Nonmotile, pleomorphic rod-shaped, gram-negative bacilli*
- This description might fit certain bacteria like **_Haemophilus influenzae_**, which typically causes **respiratory tract infections or meningitis**, not UTIs.
- While *E. coli* is a gram-negative rod, it is **motile**, distinguishing it from this option.
Question 65: A 51-year-old man comes to the physician because of a 4-day history of fever and cough productive of foul-smelling, dark red, gelatinous sputum. He has smoked 1 pack of cigarettes daily for 30 years and drinks two 12-oz bottles of beer daily. An x-ray of the chest shows a cavity with air-fluid levels in the right lower lobe. Sputum culture grows gram-negative rods. Which of the following virulence factors is most likely involved in the pathogenesis of this patient's condition?
A. IgA protease
B. Exotoxin A
C. Capsular polysaccharide (Correct Answer)
D. P-fimbriae
E. Heat-stable toxin
Explanation: ***Capsular polysaccharide***
- The patient's symptoms (fever, foul-smelling sputum, cavitation with air-fluid levels) and risk factors (smoking) suggest a **lung abscess** likely caused by **_Klebsiella pneumoniae_**.
- **Capsular polysaccharide** is a major virulence factor for _Klebsiella pneumoniae_, providing resistance to phagocytosis and contributing to its invasive potential.
*IgA protease*
- **IgA protease** is a virulence factor produced by bacteria such as _Neisseria gonorrhoeae_, _Neisseria meningitidis_, and _Haemophilus influenzae_ to cleave IgA antibodies.
- While important for mucosal infections, it is not characteristic of the severe lung pathology described, nor a primary virulence factor for a gram-negative rod causing lung abscesses like _Klebsiella_.
*Exotoxin A*
- **Exotoxin A** is a potent exotoxin produced by _Pseudomonas aeruginosa_, which inhibits protein synthesis by ADP-ribosylation of elongation factor 2.
- While _Pseudomonas_ can cause lung infections in compromised patients, the classic description of dark red, gelatinous sputum and the strong association with gram-negative rods causing lung abscesses points more directly to _Klebsiella_.
*P-fimbriae*
- **P-fimbriae** (pyelonephritis-associated fimbriae) are adhesion factors found on uropathogenic _E. coli_, enabling them to bind to uroepithelial cells and cause urinary tract infections.
- These fimbriae are not relevant to the pathogenesis of a lung abscess caused by gram-negative rods in this clinical context.
*Heat-stable toxin*
- **Heat-stable toxin** is typically associated with enterotoxigenic _E. coli_ (ETEC), causing watery diarrhea by activating guanylate cyclase.
- This toxin is involved in gastrointestinal infections and has no role in the pathogenesis of a lung abscess.
Question 66: An 18-year-old female returning from a trip to a developing country presents with diarrhea and pain in the abdominal region. Microscopic evaluation of the stool reveals the presence of RBC's and WBC's. The patient reports poor sewage sanitation in the region she visited. The physician suspects a bacterial infection and culture reveals Gram-negative rods that are non-lactose fermenting. The A subunit of the bacteria's toxin acts to:
A. Inhibit exocytosis of ACh from synaptic terminals
B. Lyse red blood cells
C. Prevent phagocytosis
D. Act as an N-glycosidase on 28S rRNA of the 60S ribosome (Correct Answer)
E. ADP-ribosylate the Gs protein
Explanation: ***Act as an N-glycosidase on 28S rRNA of the 60S ribosome***
- The clinical presentation (bloody diarrhea, abdominal pain, exposure to poor sanitation), microscopic findings (RBCs and WBCs in stool), and bacterial characteristics (Gram-negative, non-lactose fermenting rods) point to an infection by **Shigella dysenteriae**.
- The **Shiga toxin** produced by *Shigella dysenteriae* is an A-B toxin where the A subunit acts as an **N-glycosidase**, cleaving an adenine residue from the **28S rRNA of the 60S ribosomal subunit**, thereby irreversibly inhibiting protein synthesis and causing cell death.
*Inhibit exocytosis of ACh from synaptic terminals*
- This mechanism is characteristic of **botulinum toxin**, produced by *Clostridium botulinum*, which primarily affects the nervous system, leading to **flaccid paralysis**, not bloody diarrhea.
- The clinical picture and stool findings are inconsistent with botulism.
*Lyse red blood cells*
- While some bacterial toxins are **hemolysins** and can lyse red blood cells, this is not the primary mechanism of action for the Shiga toxin.
- The lysis of RBCs observed in the stool is due to damage to the intestinal lining, not direct lysis by the toxin in the bloodstream.
*Prevent phagocytosis*
- Many bacterial capsules (e.g., *Streptococcus pneumoniae*) and surface components prevent phagocytosis, but this is a mechanism for evading the immune system, not the direct action of a toxin causing bloody diarrhea by inhibiting protein synthesis.
- The question specifically asks about the A subunit's action, which is enzymatic and intracellular.
*ADP-ribosylate the Gs protein*
- This is the mechanism of action for **cholera toxin** (produced by *Vibrio cholerae*) and **heat-labile enterotoxin** (produced by *Enterotoxigenic E. coli*).
- This action leads to continuous activation of **adenylate cyclase** and increased cAMP, resulting in severe **watery diarrhea**, not bloody diarrhea.
