Pseudomonas aeruginosa US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Pseudomonas aeruginosa. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Pseudomonas aeruginosa US Medical PG Question 1: A 62-year-old woman presents to the emergency department for evaluation of a spreading skin infection that began from an ulcer on her foot. The patient has type 2 diabetes mellitus that is poorly controlled. On examination, there is redness and erythema to the lower limb with skin breakdown along an extensive portion of the leg. The patient’s tissues separate readily from the fascial plane, prompting a diagnosis of necrotizing fasciitis. What is the exotoxin most likely associated with this patient’s presentation?
- A. Streptococcal pyogenic exotoxin A
- B. TSST-1
- C. Diphtheria toxin
- D. Exfoliative toxin
- E. Streptococcal pyogenic exotoxin B (Correct Answer)
Pseudomonas aeruginosa Explanation: ***Streptococcal pyogenic exotoxin B***
- **Streptococcal pyogenic exotoxin B** is a **cysteine protease** that directly degrades tissue, including collagen and fibronectin, leading to the rapid tissue destruction characteristic of **necrotizing fasciitis**.
- This exotoxin is frequently associated with **Group A Streptococcus (GAS)** infections, a common cause of severe soft tissue infections, especially in immunocompromised individuals like diabetics.
*Streptococcal pyogenic exotoxin A*
- This exotoxin acts as a **superantigen**, primarily causing symptoms of **streptococcal toxic shock syndrome** (STSS), characterized by fever, rash, and organ failure.
- While GAS can cause necrotizing fasciitis, Exotoxin A is more closely linked to toxic shock phenomena rather than direct tissue destruction.
*TSST-1*
- **Toxic Shock Syndrome Toxin-1 (TSST-1)** is produced by **Staphylococcus aureus** and is a classic cause of **staphylococcal toxic shock syndrome**.
- It acts as a **superantigen** but is not directly responsible for the extensive tissue necrosis seen in necrotizing fasciitis caused by streptococci.
*Diphtheria toxin*
- **Diphtheria toxin**, produced by *Corynebacterium diphtheriae*, inhibits **protein synthesis** by inactivating elongation factor-2 (EF-2), leading to cell death.
- It causes diphtheria, characterized by a **pseudomembrane** in the throat and myocarditis, not necrotizing fasciitis.
*Exfoliative toxin*
- **Exfoliative toxins A and B** are produced by **Staphylococcus aureus** and are responsible for **Staphylococcal Scalded Skin Syndrome (SSSS)**.
- These toxins cause cleavage of desmoglein-1 in the epidermis, leading to widespread blistering and desquamation, not deep tissue necrosis.
Pseudomonas aeruginosa US Medical PG Question 2: A 65-year-old man presents with low-grade fever and malaise for the last 4 months. He also says he has lost 9 kg (20 lb) during this period and suffers from extreme fatigue. Past medical history is significant for a mitral valve replacement 5 years ago. His temperature is 38.1°C (100.6°F), respirations are 22/min, pulse is 102/min, and blood pressure is 138/78 mm Hg. On physical examination, there is a new onset 2/6 holosystolic murmur loudest in the apical area of the precordium. Which of the following organisms is the most likely cause of this patient’s condition?
- A. Enterococcus (Correct Answer)
- B. Candida albicans
- C. Coagulase-negative Staphylococcus spp.
- D. Escherichia coli
- E. Pseudomonas aeruginosa
Pseudomonas aeruginosa Explanation: ***Enterococcus***
- This patient has **late prosthetic valve endocarditis (PVE)**, occurring **5 years after mitral valve replacement**.
- Late PVE (>1 year post-surgery) is most commonly caused by **viridans streptococci** and ***Staphylococcus aureus***, followed by **Enterococcus species**.
- Among the given options, ***Enterococcus*** is the most common cause, particularly in **elderly patients**.
- The **subacute presentation** with **4 months of low-grade fever, malaise, weight loss**, and **new-onset murmur** is consistent with enterococcal endocarditis.
- Enterococcus is a common cause of healthcare-associated endocarditis and has increased prevalence in patients with prosthetic valves.
*Coagulase-negative Staphylococcus spp.*
- Coagulase-negative staphylococci (e.g., *S. epidermidis*) are the **most common cause of early PVE** (within the first year after surgery).
- At **5 years post-surgery**, this represents **late PVE**, where coagulase-negative staph is much less common than streptococci, *S. aureus*, and enterococci.
- While it can occur in late PVE, it is not the most likely organism in this timeframe.
*Escherichia coli*
- *E. coli* is an uncommon cause of endocarditis, typically associated with underlying gastrointestinal or urinary tract sources.
- It generally presents **acutely** rather than with the subacute 4-month course seen here.
- Not a typical cause of prosthetic valve endocarditis.
*Candida albicans*
- Fungal endocarditis is rare and typically seen in **immunocompromised patients, IV drug users**, or those with **prolonged ICU stays** with indwelling catheters.
- While *Candida* can cause PVE, it is much less common than bacterial causes in this clinical context.
*Pseudomonas aeruginosa*
- *Pseudomonas* endocarditis typically occurs in **IV drug users** and commonly affects the **tricuspid valve** (right-sided).
- Usually presents as an **acute infection** rather than the subacute presentation here.
- Not a common cause of late prosthetic valve endocarditis in non-IVDU patients.
Pseudomonas aeruginosa US Medical PG Question 3: A microbiology student was given a swab containing an unknown bacteria taken from the wound of a soldier and asked to identify the causative agent. She determined that the bacteria was a gram-positive, spore-forming bacilli, but had difficulty narrowing it down to the specific bacteria. The next test she performed was the Nagler's test, in which she grew the bacteria on a plate made from egg yolk, which would demonstrate the ability of the bacteria to hydrolyze phospholipids and produce an area of opacity. Half the plate contained a specific antitoxin which prevented hydrolysis of phospholipids while the other half did not contain any antitoxin. The bacteria produced an area of opacity only on half of the plate containing no antitoxin. Which of the following toxins was the antitoxin targeting?
- A. Alpha toxin (Correct Answer)
- B. Exotoxin A
- C. Tetanus toxin
- D. Diphtheria toxin
- E. Botulinum toxin
Pseudomonas aeruginosa Explanation: ***Alpha toxin***
- The scenario describes a **Nagler's test**, which is specifically used to detect the presence of **alpha toxin (lecithinase)** produced by *Clostridium perfringens*.
- The antitoxin prevents the hydrolysis of phospholipids and the formation of opacity, confirming that the opacity is due to the alpha toxin.
*Exotoxin A*
- **Exotoxin A** is a toxin produced by *Pseudomonas aeruginosa* and inhibits protein synthesis.
- It is not associated with the **Nagler's test** or phospholipid hydrolysis on egg yolk agar.
*Tetanus toxin*
- **Tetanus toxin** is produced by *Clostridium tetani* and causes spastic paralysis by inhibiting inhibitory neurotransmitter release.
- It is not involved in phospholipid hydrolysis or detected by the **Nagler's test**.
*Diphtheria toxin*
- **Diphtheria toxin** is produced by *Corynebacterium diphtheriae* and inhibits protein synthesis, leading to cellular death.
- This toxin is not detected by the **Nagler's test** and does not cause phospholipid hydrolysis.
*Botulinum toxin*
- **Botulinum toxin** is produced by *Clostridium botulinum* and causes flaccid paralysis by inhibiting acetylcholine release at the neuromuscular junction.
- It is not associated with the **Nagler's test** or the hydrolysis of phospholipids.
Pseudomonas aeruginosa US Medical PG Question 4: A 65-year-old woman undergoes an abdominal hysterectomy. She develops pain and discharge at the incision site on the fourth postoperative day. The past medical history is significant for diabetes of 12 years duration, which is well-controlled on insulin. Pus from the incision site is sent for culture on MacConkey agar, which shows white-colorless colonies. On blood agar, the colonies were green. Biochemical tests reveal an oxidase-positive organism. Which of the following is the most likely pathogen?
- A. Staphylococcus aureus
- B. Enterococcus faecalis
- C. Streptococcus pyogenes
- D. Pseudomonas aeruginosa (Correct Answer)
- E. Staphylococcus epidermidis
Pseudomonas aeruginosa Explanation: ***Pseudomonas aeruginosa***
- The combination of **white, colorless colonies on MacConkey agar** (indicating a non-lactose fermenter), **green colonies on blood agar** (due to pigment production), and a **positive oxidase test** is highly characteristic of *Pseudomonas aeruginosa*.
- This organism is a common cause of **nosocomial infections**, particularly in immunocompromised patients (like those with diabetes) and in postoperative wound infections.
*Staphylococcus aureus*
- This bacterium would typically produce **golden-yellow colonies** on blood agar and **no growth on MacConkey agar**.
- It is **oxidase-negative** and a common cause of surgical site infections, but its colonial morphology and biochemical tests do not match the description.
*Enterococcus faecalis*
- This organism is a **Gram-positive coccus** that would not grow well on MacConkey agar and would not produce green colonies on blood agar or be oxidase-positive.
- It is a common cause of urinary tract and wound infections, especially in hospitalized patients.
*Streptococcus pyogenes*
- This is a **beta-hemolytic Streptococcus** that typically produces small, clear colonies with a zone of complete hemolysis on blood agar and would not grow on MacConkey agar.
- It is also **oxidase-negative**, making it inconsistent with the findings.
*Staphylococcus epidermidis*
- This organism forms **white colonies** on blood agar and would not grow on MacConkey agar or produce green pigment.
- It is **coagulase-negative** and **oxidase-negative**, and while it can cause surgical site infections, its colonial characteristics differ.
Pseudomonas aeruginosa US Medical PG Question 5: A 9-year-old boy with cystic fibrosis (CF) presents to the clinic with fever, increased sputum production, and cough. The vital signs include: temperature 38.0°C (100.4°F), blood pressure 126/74 mm Hg, heart rate 103/min, and respiratory rate 22/min. His physical examination is significant for short stature, thin body frame, decreased breath sounds bilateral, and a 2/6 holosystolic murmur heard best on the upper right sternal border. His pulmonary function tests are at his baseline, and his sputum cultures reveal Pseudomonas aeruginosa. What is the best treatment option for this patient?
- A. Dornase alfa 2.5 mg as a single-use
- B. Oral cephalexin for 14 days
- C. Inhaled tobramycin for 28 days (Correct Answer)
- D. Minocycline for 28 days
- E. Sulfamethoxazole and trimethoprim for 14 days
Pseudomonas aeruginosa Explanation: ***Inhaled tobramycin for 28 days***
- This patient presents with a **mild pulmonary exacerbation** of **cystic fibrosis (CF)**, characterized by fever, increased sputum production, and the isolation of **Pseudomonas aeruginosa** from sputum cultures.
- The **pulmonary function tests (PFTs) at baseline** indicate this is a **mild exacerbation** that can be managed in the **outpatient setting**.
- **Inhaled tobramycin** is the appropriate first-line treatment for **mild-to-moderate exacerbations** and for **chronic suppressive therapy** of **Pseudomonas aeruginosa** infections in CF patients.
- It is effective in improving lung function, reducing bacterial load, and decreasing exacerbation frequency with minimal systemic toxicity.
- More severe exacerbations (significant PFT decline, respiratory distress) would require **IV antipseudomonal antibiotics**.
*Dornase alfa 2.5 mg as a single-use*
- **Dornase alfa** (DNase) is a mucolytic agent used in CF to reduce sputum viscosity and improve airway clearance by breaking down extracellular DNA in mucus.
- While beneficial for **chronic airway clearance therapy**, it is not an antibiotic and does not directly treat the **bacterial infection** causing the current exacerbation.
*Oral cephalexin for 14 days*
- **Cephalexin** is a first-generation cephalosporin that primarily targets **gram-positive bacteria** (such as Staphylococcus aureus) and some **gram-negative bacteria**.
- It is **completely ineffective against Pseudomonas aeruginosa**, which is intrinsically resistant to first-generation cephalosporins.
*Minocycline for 28 days*
- **Minocycline** is a **tetracycline antibiotic** with activity against many bacteria, including some **atypical pathogens** and **Staphylococcus aureus**.
- However, it is **not effective against Pseudomonas aeruginosa**, which is intrinsically resistant to tetracyclines and is a common and aggressive pathogen in CF patients.
*Sulfamethoxazole and trimethoprim for 14 days*
- **Sulfamethoxazole and trimethoprim** (TMP-SMX, Bactrim) is an antibiotic combination effective against various bacteria, including some **gram-negative organisms** and **Staphylococcus aureus**.
- It does **not provide adequate coverage for Pseudomonas aeruginosa**, making it an inappropriate choice for this patient's documented infection.
Pseudomonas aeruginosa US Medical PG Question 6: Blood cultures are sent to the laboratory. Intravenous antibiotic therapy is started. Transesophageal echocardiography shows a large, oscillating vegetation attached to the tricuspid valve. There are multiple small vegetations attached to tips of the tricuspid valve leaflets. There is moderate tricuspid regurgitation. The left side of the heart and the ejection fraction are normal. Which of the following is the most likely causal organism of this patient's condition?
- A. Streptococcus sanguinis
- B. Staphylococcus aureus (Correct Answer)
- C. Enterococcus faecalis
- D. Neisseria gonorrhoeae
- E. Staphylococcus epidermidis
Pseudomonas aeruginosa Explanation: ***Staphylococcus aureus***
- **_Staphylococcus aureus_** is the most common cause of **acute infective endocarditis**, particularly in intravenous drug users, which often affects the **tricuspid valve**.
- The presence of large, oscillating vegetations and **multiple small vegetations** on the tricuspid valve strongly suggests an aggressive infection, typical of _S. aureus_.
*Streptococcus sanguinis*
- _Streptococcus sanguinis_ is a common cause of **subacute infective endocarditis** in patients with pre-existing valvular disease but rarely causes acute, aggressive right-sided endocarditis.
- It's typically associated with **dental procedures** and usually affects the left side of the heart.
*Enterococcus faecalis*
- _Enterococcus faecalis_ can cause endocarditis, often associated with **genitourinary or gastrointestinal procedures**, and typically affects older men.
- While it can cause virulent endocarditis, it is less commonly associated with acute right-sided disease in this demographic compared to _S. aureus_.
*Neisseria gonorrhoeae*
- **_Neisseria gonorrhoeae_** is a rare cause of endocarditis, usually seen in younger, sexually active individuals, and often involves the aortic valve.
- While it can be acute, it is an extremely uncommon cause of **tricuspid valve endocarditis**.
*Staphylococcus epidermidis*
- **_Staphylococcus epidermidis_** is primarily associated with **prosthetic valve endocarditis** or foreign bodies, often presenting as a subacute infection.
- It rarely causes natural valve endocarditis, especially acute right-sided disease in this context.
Pseudomonas aeruginosa US Medical PG Question 7: A 13-year-old boy is brought by his mother to the emergency department because he has had fever, chills, and severe coughing for the last two days. While they originally tried to manage his condition at home, he has become increasingly fatigued and hard to arouse. He has a history of recurrent lung infections and occasionally has multiple foul smelling stools. On presentation, his temperature is 102.2 °F (39 °C), blood pressure is 106/71 mmHg, pulse is 112/min, and respirations are 20/min. Physical exam reveals scattered rhonchi over both lung fields, rales at the base of the right lung base and corresponding dullness to percussion. The most likely organism responsible for this patient's symptoms has which of the following characteristics?
- A. Mixed anaerobic rods
- B. Lancet-shaped diplococci
- C. Mucoid lactose-fermenting rod
- D. Green gram-negative rod (Correct Answer)
- E. Coagulase-positive, gram-positive cocci
Pseudomonas aeruginosa Explanation: ***Green gram-negative rod***
- The patient's history of **recurrent lung infections** and **foul-smelling stools (malabsorption)** is highly suggestive of **cystic fibrosis (CF)**.
- **Pseudomonas aeruginosa**, a **green gram-negative rod** (due to pyocyanin pigment), is a common cause of severe pulmonary infections in CF patients and is a significant contributor to morbidity and mortality.
*Mixed anaerobic rods*
- This typically causes **aspiration pneumonia**, often involving the posterior segments of the upper lobes or superior segments of the lower lobes.
- While patients with CF can have aspiration, the **recurrent nature** and specific **malabsorption symptoms** point more strongly to *Pseudomonas*.
*Lancet-shaped diplococci*
- This describes **Streptococcus pneumoniae**, a common cause of **community-acquired pneumonia**.
- While possible, it does not explain the recurrent infections or the patient's underlying condition of malabsorption and is less specific for CF-related pneumonia than *Pseudomonas*.
*Mucoid lactose-fermenting rod*
- This describes **Klebsiella pneumoniae**, which can cause severe pneumonia, often with **currant jelly sputum**.
- While *Klebsiella* can cause lung infections, it is not as characteristic of recurrent infections in CF patients as *Pseudomonas*, and the malabsorption connection is weaker.
*Coagulase-positive, gram-positive cocci*
- This describes **Staphylococcus aureus**, which is another common pathogen in CF, especially in younger patients.
- However, the description of a "green" gram-negative rod in the correct option points more specifically to *Pseudomonas aeruginosa*, which becomes increasingly prevalent and problematic in older CF patients.
Pseudomonas aeruginosa US Medical PG Question 8: 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
Pseudomonas aeruginosa 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.
Pseudomonas aeruginosa US Medical PG Question 9: A 20-year-old woman presents for a follow-up visit with her physician. She has a history of cystic fibrosis and is currently under treatment. She has recently been struggling with recurrent bouts of cough and foul-smelling, mucopurulent sputum over the past year. Each episode lasts for about a week or so and then subsides. She does not have a fever or chills during these episodes. She has been hospitalized several times for pneumonia as a child and continues to struggle with diarrhea. Physically she appears to be underweight and in distress. Auscultation reveals reduced breath sounds on the lower lung fields with prominent rhonchi. Which of the following infectious agents is most likely associated with the recurrent symptoms this patient is experiencing?
- A. Mycobacterium avium
- B. Pseudomonas (Correct Answer)
- C. Histoplasma
- D. Pneumococcus
- E. Listeria
Pseudomonas aeruginosa Explanation: ***Pseudomonas***
- **Pseudomonas aeruginosa** is a common and opportunistic pathogen in patients with **cystic fibrosis** due to altered mucus secretion and impaired mucociliary clearance.
- Recurrent cough, foul-smelling, and **mucopurulent sputum** are classic symptoms of **Pseudomonas** lung infections in CF patients, often leading to chronic colonization and bronchiectasis.
*Mycobacterium avium*
- While *Mycobacterium avium complex* (MAC) can infect patients with cystic fibrosis, it typically causes a **more indolent and chronic lung disease** rather than recurrent, self-limiting bouts of cough and sputum.
- MAC infections are often associated with **nodular or cavitary lesions** on imaging and may require prolonged multidrug therapy.
*Histoplasmosis*
- **Histoplasmosis** is a fungal infection endemic to certain geographic regions (e.g., Ohio and Mississippi River valleys) and is acquired by inhaling spores.
- It's **not a typical or recurrent pathogen** in cystic fibrosis patients in the way bacterial infections are, and its presentation often includes fever, chills, and disseminated disease in immunocompromised individuals.
*Pneumococcus*
- *Streptococcus pneumoniae* (**Pneumococcus**) is a common cause of **acute bacterial pneumonia** in the general population, including young children.
- While CF patients can get pneumococcal infections, the pattern of **recurrent bouts of foul-smelling mucopurulent sputum** without fever and the chronic nature of the lung disease point away from typical acute pneumococcal infection and more towards a chronic colonizer like *Pseudomonas*.
*Listeria*
- *Listeria monocytogenes* is primarily a cause of **foodborne illness**, leading to gastroenteritis, meningitis, or sepsis, particularly in immunocompromised individuals, pregnant women, and neonates.
- It is **not a common respiratory pathogen**, and its presentation does not align with the described recurrent pulmonary symptoms in a cystic fibrosis patient.
Pseudomonas aeruginosa US Medical PG Question 10: An 8-year-old boy is brought to the emergency department because of a 4-day history of severe, left-sided ear pain and purulent discharge from his left ear. One week ago, he returned with his family from their annual summer vacation at a lakeside cabin, where he spent most of the time outdoors hiking and swimming. Examination shows tragal tenderness and a markedly edematous and erythematous external auditory canal. Audiometry shows conductive hearing loss of the left ear. Which of the following is the most likely cause of this patient's symptoms?
- A. Pleomorphic replacement of normal bone
- B. Abnormal epithelial growth on tympanic membrane
- C. Infection with Pseudomonas aeruginosa (Correct Answer)
- D. Infection with varicella zoster virus
- E. Infection with Aspergillus species
Pseudomonas aeruginosa Explanation: ***Infection with Pseudomonas aeruginosa***
- The patient's history of swimming, followed by severe ear pain, purulent discharge, tragal tenderness, and an edematous external auditory canal, are classic signs of **otitis externa** (swimmer's ear).
- **Pseudomonas aeruginosa** is the most common bacterial cause of otitis externa, thriving in moist environments.
*Pleomorphic replacement of normal bone*
- This describes features more consistent with a **bony tumor** or a severe, chronic infection leading to bone erosion, which is not typically seen in acute otitis externa.
- While otitis externa can become severe, the initial presentation here strongly points to an acute infectious process rather than a neoplastic transformation.
*Abnormal epithelial growth on tympanic membrane*
- This description suggests a **cholesteatoma**, which is an abnormal skin growth behind the eardrum.
- Cholesteatomas typically cause hearing loss and chronic ear discharge but are not usually associated with the acute onset of severe pain and tragal tenderness typical of otitis externa.
*Infection with varicella zoster virus*
- An infection with **varicella zoster virus** in the ear would cause **Ramsay Hunt syndrome**, characterized by severe ear pain, vesicular rash on the ear or in the auditory canal, facial paralysis, and hearing loss.
- The absence of a vesicular rash and facial paralysis makes this diagnosis unlikely.
*Infection with Aspergillus species*
- While fungal infections (like **otomycosis** caused by Aspergillus) can occur in the ear, they are less common than bacterial infections and typically present with symptoms such as itching, aural fullness, and a white or black fungal debris.
- The described purulent discharge and severe pain are more characteristic of a bacterial infection.
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