A 37-year-old woman with a history of anorectal abscesses complains of pain in the perianal region. Physical examination reveals mild swelling, tenderness, and erythema of the perianal skin. She is prescribed oral ampicillin and asked to return for follow-up. Two days later, the patient presents with a high-grade fever, syncope, and increased swelling. Which of the following would be the most common mechanism of resistance leading to the failure of antibiotic therapy in this patient?
Q2
A 42-year-old woman with a history of multiple sclerosis and recurrent urinary tract infections comes to the emergency department because of flank pain and fever. Her temperature is 38.8°C (101.8°F). Examination shows left-sided costovertebral angle tenderness. She is admitted to the hospital and started on intravenous vancomycin. Three days later, her symptoms have not improved. Urine culture shows growth of Enterococcus faecalis. Which of the following best describes the most likely mechanism of antibiotic resistance in this patient?
Q3
A team of intensivists working in a private intensive care unit (ICU) observe that the clinical efficacy of vancomycin is low, and proven nosocomial infections have increased progressively over the past year. A clinical microbiologist is invited to conduct a bacteriological audit of the ICU. He analyzes the microbiological reports of all patients treated with vancomycin over the last 2 years and takes relevant samples from the ICU for culture and antibiotic sensitivity analysis. The audit concludes that there is an increased incidence of vancomycin-resistant Enterococcus fecalis infections. Which of the following mechanisms best explains the changes that took place in the bacteria?
Q4
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?
Q5
A 47-year-old woman comes to the physician because of a 6-week history of fatigue and low-grade fever. She has no history of serious illness except for a bicuspid aortic valve, diagnosed 10 years ago. She does not use illicit drugs. Her temperature is 37.7°C (99.9°F). Physical examination shows petechiae under the fingernails and multiple tender, red nodules on the fingers. A new grade 2/6 diastolic murmur is heard at the right second intercostal space. Which of the following is the most likely causal organism?
Q6
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?
Q7
A 63-year-old female recovering from a total shoulder arthroplasty completed 6 days ago presents complaining of joint pain in her repaired shoulder. Temperature is 39 degrees Celsius. Physical examination demonstrates erythema and significant tenderness around the incision site. Wound cultures reveal Gram-positive cocci that are resistant to nafcillin. Which of the following organisms is the most likely cause of this patient's condition?
Q8
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?
Q9
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?
Q10
A 7-year-old boy with a history of cystic fibrosis is brought to the physician for evaluation of recurrent episodes of productive cough, wheezing, and shortness of breath over the past month. Physical examination shows coarse crackles and expiratory wheezing over both lung fields. Serum studies show elevated levels of IgE and eosinophilia. A CT scan of the lungs shows centrally dilated bronchi with thickened walls and peripheral airspace consolidation. Antibiotic therapy is initiated. One week later, the patient continues to show deterioration in lung function. A sputum culture is most likely to grow which of the following?
Biofilm-associated antimicrobial resistance US Medical PG Practice Questions and MCQs
Question 1: A 37-year-old woman with a history of anorectal abscesses complains of pain in the perianal region. Physical examination reveals mild swelling, tenderness, and erythema of the perianal skin. She is prescribed oral ampicillin and asked to return for follow-up. Two days later, the patient presents with a high-grade fever, syncope, and increased swelling. Which of the following would be the most common mechanism of resistance leading to the failure of antibiotic therapy in this patient?
A. Intrinsic absence of a target site for the drug
B. Use of an altered metabolic pathway
C. Production of beta-lactamase enzyme (Correct Answer)
D. Altered structural target for the drug
E. Drug efflux pump
Explanation: ***Production of beta-lactamase enzyme***
- The patient's symptoms of a rapidly worsening infection despite ampicillin treatment suggest the presence of a **beta-lactamase producing organism**. Ampicillin is a **beta-lactam antibiotic** that is inactivated by these enzymes.
- Anorectal abscesses and rapidly progressing soft tissue infections are often caused by **polymicrobial flora**, including staphylococci and enterococci, many of which can produce **beta-lactamase**.
*Intrinsic absence of a target site for the drug*
- While some bacteria inherently lack the target site for certain drugs (e.g., mycoplasma lacking a cell wall, thus being resistant to beta-lactams), this is less likely to be the **most common mechanism of acquired resistance** leading to treatment failure in a typical perianal infection.
- The rapid progression and failed initial treatment point towards an **acquired mechanism of resistance** rather than an intrinsic one.
*Use of an altered metabolic pathway*
- This mechanism, such as altered **folate synthesis pathways** in resistance to trimethoprim-sulfamethoxazole, is less common as the primary mechanism for ampicillin resistance.
- Ampicillin's mechanism of action primarily targets the **bacterial cell wall**, not a metabolic pathway in the same way.
*Altered structural target for the drug*
- This involves modifications to the **penicillin-binding proteins (PBPs)**, which are the targets of beta-lactam antibiotics like ampicillin. While a valid mechanism (e.g., in MRSA), the **production of beta-lactamase** is generally a more widespread and common cause of ampicillin failure, especially in infections involving mixed flora from the perianal region.
- Given the abrupt failure of ampicillin, **beta-lactamase inactivation** is a more immediate and common cause than a rapid mutational change in PBPs.
*Drug efflux pump*
- **Efflux pumps** actively remove antibiotics from the bacterial cell, contributing to resistance against various drug classes.
- While efflux pumps can play a role, the dominant mechanism for resistance to **ampicillin** in many common perianal pathogens is the **enzymatic degradation by beta-lactamases**.
Question 2: A 42-year-old woman with a history of multiple sclerosis and recurrent urinary tract infections comes to the emergency department because of flank pain and fever. Her temperature is 38.8°C (101.8°F). Examination shows left-sided costovertebral angle tenderness. She is admitted to the hospital and started on intravenous vancomycin. Three days later, her symptoms have not improved. Urine culture shows growth of Enterococcus faecalis. Which of the following best describes the most likely mechanism of antibiotic resistance in this patient?
A. Increased efflux across bacterial cell membranes
B. Production of beta-lactamase
C. Alteration of penicillin-binding proteins
D. Alteration of peptidoglycan synthesis (Correct Answer)
E. Alteration of ribosomal targets
Explanation: ***Alteration of peptidoglycan synthesis***
- **Vancomycin** targets the **D-Ala-D-Ala terminus** on the peptidoglycan precursor, preventing cross-linking during bacterial cell wall synthesis.
- **Vancomycin resistance in Enterococcus faecalis** occurs through acquisition of resistance genes (vanA, vanB) that encode enzymes modifying the peptidoglycan precursor from **D-Ala-D-Ala to D-Ala-D-Lac**.
- This structural change reduces vancomycin's binding affinity by approximately 1000-fold, rendering the antibiotic ineffective.
- The mechanism directly involves **alteration of the peptidoglycan synthesis pathway**, specifically the terminal amino acid residues of the pentapeptide precursor.
*Increased efflux across bacterial cell membranes*
- This mechanism involves **efflux pumps that actively transport antibiotics out of the bacterial cell**, reducing intracellular concentration.
- While efflux pumps contribute to resistance for antibiotics like **tetracyclines, fluoroquinolones, and macrolides**, this is not the primary mechanism of vancomycin resistance in Enterococcus.
*Production of beta-lactamase*
- **Beta-lactamase enzymes** hydrolyze the **beta-lactam ring** of antibiotics like **penicillins and cephalosporins**, rendering them inactive.
- **Vancomycin is a glycopeptide antibiotic, not a beta-lactam**, so its efficacy is not affected by beta-lactamase production.
*Alteration of ribosomal targets*
- This mechanism confers resistance to antibiotics that target **bacterial ribosomes** to inhibit protein synthesis, such as **macrolides, aminoglycosides, and tetracyclines**.
- **Vancomycin acts on cell wall synthesis**, not protein synthesis, so alteration of ribosomal targets is not relevant to vancomycin resistance.
*Alteration of penicillin-binding proteins*
- **Penicillin-binding proteins (PBPs)** are the targets of **beta-lactam antibiotics** (penicillins, cephalosporins, carbapenems).
- Alterations in PBPs cause resistance to beta-lactams, not to vancomycin.
- **Vancomycin does not interact with PBPs**; it binds directly to the D-Ala-D-Ala terminus of peptidoglycan precursors in the cell wall.
Question 3: A team of intensivists working in a private intensive care unit (ICU) observe that the clinical efficacy of vancomycin is low, and proven nosocomial infections have increased progressively over the past year. A clinical microbiologist is invited to conduct a bacteriological audit of the ICU. He analyzes the microbiological reports of all patients treated with vancomycin over the last 2 years and takes relevant samples from the ICU for culture and antibiotic sensitivity analysis. The audit concludes that there is an increased incidence of vancomycin-resistant Enterococcus fecalis infections. Which of the following mechanisms best explains the changes that took place in the bacteria?
A. Decreased number of porins in the bacterial cell wall leading to decreased intracellular entry of the antibiotic
B. Production of an enzyme that hydrolyzes the antibiotic
C. Protection of the antibiotic-binding site by Qnr protein
D. Increased expression of efflux pumps which extrude the antibiotic from the bacterial cell
E. Replacement of the terminal D-Ala in the cell wall peptidoglycan by D-lactate (Correct Answer)
Explanation: ***Replacement of the terminal D-ala in the cell wall peptidoglycan by D-lactate***
- **Vancomycin** exerts its antibacterial effect by binding to the **D-Ala-D-Ala** terminus of the peptidoglycan precursor in the bacterial cell wall, preventing its incorporation.
- In **vancomycin-resistant Enterococcus (VRE)**, the D-Ala-D-Ala is replaced by **D-Ala-D-Lac**, which significantly reduces vancomycin's binding affinity, leading to resistance.
*Decreased number of porins in the bacterial cell wall leading to decreased intracellular entry of the antibiotic*
- This mechanism primarily affects **Gram-negative bacteria**, where porins are crucial for antibiotic entry through the outer membrane.
- **Enterococcus faecalis** is a **Gram-positive bacterium** and does not rely on porins in the same way for vancomycin uptake.
*Production of an enzyme that hydrolyzes the antibiotic*
- This mechanism is characteristic of resistance to **beta-lactam antibiotics** (e.g., penicillinases, cephalosporinases).
- Vancomycin is not a beta-lactam, and its resistance mechanism in Enterococcus does not typically involve enzymatic hydrolysis.
*Protection of the antibiotic-binding site by Qnr protein*
- **Qnr proteins** are associated with **quinolone resistance**, specifically by protecting DNA gyrase and topoisomerase IV from quinolone inhibition.
- This mechanism is irrelevant to vancomycin, which targets the bacterial cell wall.
*Increased expression of efflux pumps which extrude the antibiotic from the bacterial cell*
- Efflux pumps are a common mechanism of antibiotic resistance against a wide range of antibiotics, including **tetracyclines, macrolides, and fluoroquinolones**.
- While efflux pumps can contribute to some forms of resistance, they are not the primary or best-explained mechanism for **high-level vancomycin resistance in Enterococcus**.
Question 4: 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
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.
Question 5: A 47-year-old woman comes to the physician because of a 6-week history of fatigue and low-grade fever. She has no history of serious illness except for a bicuspid aortic valve, diagnosed 10 years ago. She does not use illicit drugs. Her temperature is 37.7°C (99.9°F). Physical examination shows petechiae under the fingernails and multiple tender, red nodules on the fingers. A new grade 2/6 diastolic murmur is heard at the right second intercostal space. Which of the following is the most likely causal organism?
A. Staphylococcus epidermidis
B. Streptococcus pyogenes
C. Streptococcus sanguinis (Correct Answer)
D. Streptococcus pneumoniae
E. Enterococcus faecalis
Explanation: ***Streptococcus sanguinis***
- The patient's presentation with **fatigue, low-grade fever, petechiae, tender nodules (Osler nodes)**, and a **new diastolic murmur** in a patient with a **bicuspid aortic valve** is highly suggestive of **infective endocarditis**.
- **Streptococcus sanguinis** (and other viridans streptococci) are common causes of subacute bacterial endocarditis, often associated with **oral flora** and pre-existing valvular heart disease.
*Staphylococcus epidermidis*
- This organism is a common cause of **prosthetic valve endocarditis** and **nosocomial infections** but is less likely to cause endocarditis in a native valve without a history of recent surgery or intravenous lines.
- While it can cause endocarditis, the clinical features here, especially the lack of recent medical interventions, point away from *S. epidermidis* as the primary cause.
*Streptococcus pyogenes*
- **Streptococcus pyogenes** is primarily known for causing **strep throat, scarlet fever, and rheumatic fever**, which can lead to rheumatic heart disease but rarely causes acute or subacute infective endocarditis directly.
- It typically causes more acute and severe infections, which doesn't align with the 6-week history of low-grade fever and fatigue.
*Streptococcus pneumoniae*
- **Streptococcus pneumoniae** is a common cause of **pneumonia, meningitis, and otitis media** but is an uncommon cause of infective endocarditis, accounting for a very small percentage of cases.
- Endocarditis due to *S. pneumoniae* tends to be **acute and fulminant**, often associated with severe systemic illness, which is not fully consistent with the subacute presentation here.
*Enterococcus faecalis*
- **Enterococcus faecalis** is a common cause of **nosocomial urinary tract infections** and can cause endocarditis, especially in older patients or those with gastrointestinal or genitourinary procedures.
- While it's a possibility for endocarditis, the oral flora association with viridans streptococci (like *S. sanguinis*) in the context of a bicuspid aortic valve makes it a more direct fit.
Question 6: 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
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.
Question 7: A 63-year-old female recovering from a total shoulder arthroplasty completed 6 days ago presents complaining of joint pain in her repaired shoulder. Temperature is 39 degrees Celsius. Physical examination demonstrates erythema and significant tenderness around the incision site. Wound cultures reveal Gram-positive cocci that are resistant to nafcillin. Which of the following organisms is the most likely cause of this patient's condition?
A. Streptococcus pyogenes
B. Escherichia coli
C. Streptococcus viridans
D. Staphylococcus epidermidis
E. Staphylococcus aureus (Correct Answer)
Explanation: ***Staphylococcus aureus***
- The combination of **post-surgical infection**, **erythema**, and fever with **Gram-positive cocci** that are **nafcillin-resistant** is highly indicative of **Methicillin-Resistant Staphylococcus aureus (MRSA)**.
- *S. aureus* is a common cause of **surgical site infections**, and its resistance to nafcillin implies it is MRSA, a significant clinical concern for its difficulty in treatment.
*Streptococcus pyogenes*
- While *S. pyogenes* is a Gram-positive coccus that can cause skin and soft tissue infections, it is typically **susceptible to penicillin** and related antibiotics like nafcillin, unlike the organism described.
- It is more commonly associated with **streptococcal pharyngitis** or **cellulitis**, and while it can cause severe disease, its resistance profile doesn't match the clinical picture.
*Escherichia coli*
- *E. coli* is a **Gram-negative rod**, not a Gram-positive coccus.
- It is a common cause of **urinary tract infections** and **gastrointestinal infections**, making it an unlikely pathogen for a post-surgical joint infection unless contaminated from a visceral source.
*Streptococcus viridans*
- **Viridans streptococci** are Gram-positive cocci but are typically associated with **endocarditis** or dental infections, especially after poor dental hygiene or procedures.
- They are usually **susceptible to penicillin** and do not typically exhibit nafcillin resistance as the primary feature in a post-arthroplasty infection.
*Staphylococcus epidermidis*
- *S. epidermidis* is a **coagulase-negative Staphylococcus** known for forming **biofilms on prosthetic devices**, leading to chronic, low-grade infections.
- While it can be nafcillin-resistant, the **acute presentation** with fever and significant inflammation suggests a more virulent pathogen like *S. aureus*, as *S. epidermidis* infections are typically indolent.
Question 8: 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
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.
Question 9: 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 10: A 7-year-old boy with a history of cystic fibrosis is brought to the physician for evaluation of recurrent episodes of productive cough, wheezing, and shortness of breath over the past month. Physical examination shows coarse crackles and expiratory wheezing over both lung fields. Serum studies show elevated levels of IgE and eosinophilia. A CT scan of the lungs shows centrally dilated bronchi with thickened walls and peripheral airspace consolidation. Antibiotic therapy is initiated. One week later, the patient continues to show deterioration in lung function. A sputum culture is most likely to grow which of the following?
A. Monomorphic, septate hyphae that branch at acute angles (Correct Answer)
B. Monomorphic, broad, nonseptate hyphae that branch at wide angles
C. Dimorphic, broad-based budding yeast
D. Dimorphic, cigar-shaped budding yeast
E. Monomorphic, narrow budding encapsulated yeast
Explanation: **Monomorphic, septate hyphae that branch at acute angles**
- The patient's presentation with **cystic fibrosis (CF)**, recurrent respiratory symptoms, CT findings of **central bronchiectasis**, elevated **IgE**, and **eosinophilia** strongly points towards **allergic bronchopulmonary aspergillosis (ABPA)**.
- *Aspergillus fumigatus*, the causative agent of ABPA, is characterized microscopically by **monomorphic, septate hyphae that branch at acute angles**.
*Monomorphic, broad, nonseptate hyphae that branch at wide angles*
- This describes organisms like *Rhizopus* or *Mucor*, which cause **mucormycosis**.
- Mucormycosis typically affects immunocompromised individuals (e.g., diabetics, neutropenic patients) and presents as aggressive rhinocerebral or pulmonary infections, not ABPA.
*Dimorphic, broad-based budding yeast*
- This morphology is characteristic of *Blastomyces dermatitidis*, which causes **blastomycosis**.
- Blastomycosis is an endemic fungal infection often presenting with pulmonary symptoms, but it does not cause central bronchiectasis, elevated IgE, or eosinophilia in the context of CF.
*Dimorphic, cigar-shaped budding yeast*
- This morphology describes *Sporothrix schenckii*, which causes **sporotrichosis**.
- Sporotrichosis typically presents as a chronic ulcerative skin disease (rose gardener's disease) or, less commonly, pulmonary disease, but is not associated with ABPA.
*Monomorphic, narrow budding encapsulated yeast*
- This morphology is characteristic of *Cryptococcus neoformans*, which causes **cryptococcosis**.
- Cryptococcosis commonly affects immunocompromised individuals, causing meningoencephalitis or pulmonary disease, but its presentation is distinct from ABPA, lacking the allergic and bronchiectatic features described.
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