Coagulase-negative staphylococci US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Coagulase-negative staphylococci. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Coagulase-negative staphylococci US Medical PG Question 1: A 22-year-old sexually active, otherwise healthy female presents to her primary care physician complaining of several days of dysuria, frequency, urgency, and suprapubic pain. She denies fever, flank pain, vaginal itching, or vaginal bleeding/discharge. Which organism is most likely responsible for this patient's symptoms?
- A. Escherichia coli (Correct Answer)
- B. Chlamydia trachomatis
- C. Proteus mirabilis
- D. Staphylococcus saprophyticus
- E. Klebsiella pneumoniae
Coagulase-negative staphylococci Explanation: ***Escherichia coli***
- **_E. coli_** is the most common cause of **uncomplicated urinary tract infections (UTIs)**, accounting for 75-95% of cases.
- The patient's symptoms of **dysuria, frequency, urgency**, and **suprapubic pain** are classic for cystitis, a common manifestation of _E. coli_ infection.
*Chlamydia trachomatis*
- While _Chlamydia_ can cause **urethritis** with dysuria, it is often associated with vaginal discharge/bleeding and is primarily a sexually transmitted infection.
- The absence of **vaginal symptoms** makes it less likely to be the primary cause of these specific urinary symptoms in this scenario.
*Proteus mirabilis*
- **_Proteus mirabilis_** is a common cause of UTIs, particularly those associated with **struvite stones** due to its urease activity.
- While it can cause similar symptoms, it is less common than _E. coli_ in uncomplicated cystitis and often seen in complicated UTIs or those with a history of recurrent infections.
*Staphylococcus saprophyticus*
- _Staphylococcus saprophyticus_ is a common cause of UTIs in sexually active young women, but it is typically the **second most common** after _E. coli_.
- While a possibility, **_E. coli_ remains the most likely** given its high prevalence in uncomplicated cystitis.
*Klebsiella pneumoniae*
- _Klebsiella pneumoniae_ is more commonly associated with **hospital-acquired UTIs**, complicated UTIs, or infections in patients with underlying medical conditions, such as diabetes.
- It is a much less common cause of **uncomplicated community-acquired cystitis** in healthy young women compared to _E. coli_.
Coagulase-negative staphylococci US Medical PG Question 2: A 28-year-old woman comes to the physician because of a 4-day history of lower abdominal pain and pain with urination. Five months ago, she was treated for gonococcal urethritis. She recently moved in with her newlywed husband. She is sexually active with her husband and they do not use condoms. Her only medication is an oral contraceptive. Her temperature is 37.5°C (99.7°F) and blood pressure is 120/74 mm Hg. There is tenderness to palpation over the pelvic region. Pelvic examination shows a normal-appearing vulva and vagina. Laboratory studies show:
Leukocyte count 8,400/mm3
Urine
pH 6.7
Protein trace
WBC 60/hpf
Nitrites positive
Bacteria positive
Which of the following is the most likely causal organism?
- A. Enterococcus faecalis
- B. Klebsiella pneumoniae
- C. Neisseria gonorrhoeae
- D. Escherichia coli (Correct Answer)
- E. Staphylococcus saprophyticus
Coagulase-negative staphylococci Explanation: ***Escherichia coli***
- The presence of **nitrites** in the urine analysis strongly suggests a urinary tract infection caused by a **nitrate-reducing bacterium**, such as *E. coli*.
- *E. coli* is the **most common cause of uncomplicated UTIs**, especially in sexually active women, and the symptoms (lower abdominal pain, dysuria, pyuria, bacteriuria) are classic for a UTI.
*Enterococcus faecalis*
- While *Enterococcus faecalis* can cause UTIs, it is **less common** than *E. coli* in uncomplicated cases and typically **does not produce nitrites** in urine due to lacking nitrate reductase.
- It is more commonly associated with UTIs in hospitalized patients or those with urinary tract abnormalities.
*Klebsiella pneumoniae*
- *Klebsiella pneumoniae* can cause UTIs and is a **nitrite-producing bacterium**, but it is a **less frequent cause** of community-acquired uncomplicated UTIs compared to *E. coli*.
- It is more often associated with healthcare-associated infections or UTIs in compromised hosts.
*Neisseria gonorrhoeae*
- While the patient has a history of gonococcal urethritis, current symptoms are more consistent with a UTI, and *Neisseria gonorrhoeae* is a **rare cause of cystitis** or pyelonephritis.
- Gonorrhea primarily causes urethritis, cervicitis, or pelvic inflammatory disease, and **does not typically produce nitrites** from nitrates in urine.
*Staphylococcus saprophyticus*
- *Staphylococcus saprophyticus* is a common cause of UTIs in young, sexually active women, but it is **nitrite-negative** because it does not possess nitrate reductase.
- The positive nitrites in the urine make *E. coli* a more likely culprit in this case.
Coagulase-negative staphylococci US Medical PG Question 3: A 25-year-old woman comes to the physician because of a 2-day history of a burning sensation when urinating and increased urinary frequency. She is concerned about having contracted a sexually transmitted disease. Physical examination shows suprapubic tenderness. Urinalysis shows a negative nitrite test and positive leukocyte esterases. Urine culture grows organisms that show resistance to novobiocin on susceptibility testing. Which of the following is the most likely causal organism of this patient's symptoms?
- A. Klebsiella pneumoniae
- B. Proteus mirabilis
- C. Pseudomonas aeruginosa
- D. Staphylococcus epidermidis
- E. Staphylococcus saprophyticus (Correct Answer)
Coagulase-negative staphylococci Explanation: ***Staphylococcus saprophyticus***
- This organism is the **second most common cause of UTIs in young, sexually active women** (after *E. coli*), making it highly consistent with the patient's demographics and presentation.
- *S. saprophyticus* is characterized by **resistance to novobiocin**, which is the key laboratory test differentiating it from *S. epidermidis* (novobiocin-sensitive).
- It is **nitrite-negative** as it does not reduce nitrates to nitrites, consistent with the negative nitrite test.
*Klebsiella pneumoniae*
- While *K. pneumoniae* can cause UTIs, it is typically **nitrite-positive** because it reduces nitrates to nitrites, which contradicts the negative nitrite test result.
- This gram-negative organism would not be tested for novobiocin susceptibility, as this antibiotic is used specifically to differentiate staphylococcal species.
*Proteus mirabilis*
- *P. mirabilis* is known for causing UTIs and is **nitrite-positive** due to its ability to reduce nitrates, which is inconsistent with the patient's negative nitrite test.
- It also produces **urease**, leading to alkaline urine and struvite stones, which are not features of this acute presentation.
*Pseudomonas aeruginosa*
- *P. aeruginosa* is **nitrite-negative** (it does not reduce nitrates), which matches the test result.
- However, it is typically associated with **hospital-acquired UTIs**, catheter-related infections, or infections in immunocompromised patients, not uncomplicated community-acquired UTIs in healthy young women.
- Novobiocin testing is not routinely used for gram-negative organisms.
*Staphylococcus epidermidis*
- *S. epidermidis* is a common **skin commensal** and frequent contaminant in urine cultures.
- Critically, it is **novobiocin-sensitive**, which distinguishes it from *S. saprophyticus* and makes it incompatible with the culture findings.
- It rarely causes true UTIs unless associated with indwelling catheters or prosthetic devices.
Coagulase-negative staphylococci US Medical PG Question 4: A 64-year-old female with type 2 diabetes mellitus comes to the physician because of a 1-week history of painful red swelling on her left thigh. Examination shows a 3- x 4-cm, tender, fluctuant mass. Incision and drainage of the abscess are performed. Culture of the abscess fluid grows gram-positive, coagulase-positive cocci that are resistant to oxacillin. Which of the following best describes the mechanism of resistance of the causal organism to oxacillin?
- A. Degradation of the antibiotic
- B. Decreased uptake of the antibiotic
- C. Decreased activation of the antibiotic
- D. Altered target of the antibiotic (Correct Answer)
- E. Acetylation of the antibiotic
Coagulase-negative staphylococci Explanation: ***Altered target of the antibiotic***
- The organism described (gram-positive, coagulase-positive cocci, oxacillin-resistant) is **methicillin-resistant *Staphylococcus aureus* (MRSA)**.
- MRSA achieves oxacillin (and other beta-lactam) resistance by acquiring the ***mecA* gene**, which encodes for a **modified penicillin-binding protein (PBP2a)** with reduced affinity for beta-lactam antibiotics.
*Degradation of the antibiotic*
- This mechanism, primarily through the production of **beta-lactamase enzymes**, can degrade beta-lactam antibiotics.
- While *Staphylococcus aureus* can produce beta-lactamases, oxacillin (a **penicillinase-resistant penicillin**) is specifically engineered to be stable against these enzymes.
*Decreased uptake of the antibiotic*
- Reduced permeability of the bacterial cell wall can lead to decreased uptake, a mechanism more commonly associated with **gram-negative bacteria** due to their outer membrane.
- This is not the primary mechanism of resistance for MRSA to oxacillin.
*Decreased activation of the antibiotic*
- Some antibiotics are prodrugs that require activation by bacterial enzymes, and resistance can arise from mutations affecting this activation.
- Oxacillin is active in its administered form and does not require bacterial activation.
*Acetylation of the antibiotic*
- **Enzymatic modification**, such as acetylation, adenylylation, or phosphorylation, is a common mechanism of resistance, particularly against **aminoglycoside antibiotics**.
- This specific mechanism is not responsible for oxacillin resistance in MRSA.
Coagulase-negative staphylococci US Medical PG Question 5: A 25-year-old woman has dysuria, pyuria, increased frequency of urination, and fever of 1-day duration. She is sexually active. Urine cultures show gram-positive bacteria in clusters that are catalase-positive and coagulase-negative. The patient is started on trimethoprim-sulfamethoxazole. Which of the following characteristics is used to identify the offending organism?
- A. Resistance to bacitracin
- B. Beta hemolysis
- C. Sensitivity to novobiocin
- D. Resistance to novobiocin (Correct Answer)
- E. Sensitivity to bacitracin
Coagulase-negative staphylococci Explanation: ***Resistance to novobiocin***
- The patient's symptoms (dysuria, pyuria, frequency, fever) and urine culture results (gram-positive cocci in clusters, catalase-positive, coagulase-negative) are highly suggestive of a **Staphylococcus saprophyticus** urinary tract infection (UTI).
- A key distinguishing characteristic of **Staphylococcus saprophyticus** from other coagulase-negative staphylococci (especially *Staphylococcus epidermidis*) is its **resistance to novobiocin**.
*Resistance to bacitracin*
- This characteristic is used to differentiate Group A Streptococcus (Streptococcus pyogenes), which is sensitive to bacitracin, from other beta-hemolytic streptococci.
- However, in this case, the organism is described as gram-positive cocci in clusters and catalase-positive, indicating Staphylococcus, not Streptococcus.
*Beta hemolysis*
- While beta hemolysis is a characteristic of certain bacteria, such as *Streptococcus pyogenes* or *Staphylococcus aureus*, it is not the primary diagnostic feature for the coagulase-negative, catalase-positive staphylococcus described here.
- Furthermore, *Staphylococcus saprophyticus* is typically non-hemolytic or weakly hemolytic.
*Sensitivity to novobiocin*
- **Sensitivity to novobiocin** is a characteristic of *Staphylococcus epidermidis* and other coagulase-negative staphylococci, but not *Staphylococcus saprophyticus*.
- Since the clinical picture points strongly to *Staphylococcus saprophyticus* in a young, sexually active female with a UTI, novobiocin resistance is the expected finding.
*Sensitivity to bacitracin*
- **Sensitivity to bacitracin** is a key characteristic used to identify *Streptococcus pyogenes* (Group A Strep).
- The organism in this scenario is a gram-positive cocci in clusters and catalase-positive, indicating a Staphylococcus species, not Streptococcus.
Coagulase-negative staphylococci US Medical PG Question 6: A 24-year-old man presents with low-grade fever and shortness of breath for the last 3 weeks. Past medical history is significant for severe mitral regurgitation status post mitral valve replacement five years ago. His temperature is 38.3°C (101.0°F) and respiratory rate is 18/min. Physical examination reveals vertical hemorrhages under his nails, multiple painless erythematous lesions on his palms, and two tender, raised nodules on his fingers. Cardiac auscultation reveals a new-onset 2/6 holosystolic murmur loudest at the apex with the patient in the left lateral decubitus position. A transesophageal echocardiogram reveals vegetations on the prosthetic valve. Blood cultures reveal catalase-positive, gram-positive cocci. Which of the following characteristics is associated with the organism most likely responsible for this patient’s condition?
- A. Coagulase positive
- B. DNAse positive
- C. Hemolysis
- D. Novobiocin sensitive (Correct Answer)
- E. Optochin sensitive
Coagulase-negative staphylococci Explanation: ***Novobiocin sensitive***
- The patient has **prosthetic valve endocarditis** caused by a **catalase-positive, gram-positive coccus**, which is most likely **_Staphylococcus epidermidis_** due to its association with foreign bodies and prosthetic devices.
- _Staphylococcus epidermidis_ is a **coagulase-negative staphylococcus** that is **novobiocin sensitive**, helping to differentiate it from other coagulase-negative staphylococci like **_Staphylococcus saprophyticus_** (novobiocin resistant).
- Although this is late prosthetic valve endocarditis (5 years post-surgery), _S. epidermidis_ remains a common pathogen due to biofilm formation on prosthetic materials.
*Coagulase positive*
- **Coagulase-positive** gram-positive cocci, such as **_Staphylococcus aureus_**, are a common cause of endocarditis, especially in intravenous drug users and can also cause prosthetic valve endocarditis.
- However, the correct answer requires identifying the characteristic that differentiates the most likely organism, and **coagulase-negative** staphylococci like _S. epidermidis_ are more characteristically associated with prosthetic device infections due to their biofilm-forming capabilities.
- A positive coagulase test differentiates _S. aureus_ from coagulase-negative staphylococci.
*DNAse positive*
- **DNAse positivity** is characteristic of **_Staphylococcus aureus_** and group A beta-hemolytic streptococci (_Streptococcus pyogenes_).
- While _S. aureus_ can cause prosthetic valve endocarditis, the question asks for the characteristic most associated with the likely organism, which in the context of prosthetic devices is typically **_S. epidermidis_** (DNAse negative).
*Hemolysis*
- **Hemolysis patterns** are primarily used to differentiate **streptococcal species**, not staphylococci. For example, **beta-hemolytic streptococci** cause complete hemolysis.
- While some staphylococci can show hemolytic activity, it is not a primary characteristic used to differentiate between the most likely staphylococcal causes of prosthetic valve endocarditis.
*Optochin sensitive*
- **Optochin sensitivity** is a key characteristic used to identify **_Streptococcus pneumoniae_**.
- _S. pneumoniae_ is **catalase-negative**, while the described organism is **catalase-positive**, ruling out _S. pneumoniae_ as the causative agent.
Coagulase-negative staphylococci US Medical PG Question 7: 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
Coagulase-negative staphylococci 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.
Coagulase-negative staphylococci US Medical PG Question 8: 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
Coagulase-negative staphylococci 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.
Coagulase-negative staphylococci US Medical PG Question 9: A patient is infected with a pathogen and produces many antibodies to many antigens associated with that pathogen via Th cell-activated B cells. This takes place in the germinal center of the lymphoid tissues. If the same patient is later re-infected with the same pathogen, the immune system will respond with a much stronger response, producing antibodies with greater specificity for that pathogen in a shorter amount of time. What is the term for this process that allows the B cells to produce antibodies specific to that antigen?
- A. Affinity maturation (Correct Answer)
- B. Avidity
- C. Immunoglobulin class switching
- D. T cell negative selection
- E. T cell positive selection
Coagulase-negative staphylococci Explanation: ***Affinity maturation***
- **Affinity maturation** is the process by which B cells produce antibodies with progressively higher affinity for an antigen over the course of an immune response, allowing for a more specific and potent response upon re-exposure.
- This process occurs primarily in the **germinal centers** of lymphoid organs, driven by somatic hypermutation of antibody genes and subsequent selection of B cells exhibiting increased binding affinity.
*Avidity*
- **Avidity** refers to the overall strength of binding between a multivalent antibody and a multivalent antigen, taking into account the combined strength of multiple binding sites.
- While high avidity is a characteristic of effective antibody responses, it describes the strength of binding rather than the *process* of improving specificity and affinity over time.
*Immunoglobulin class switching*
- **Immunoglobulin class switching** (or isotype switching) is the process by which B cells change the class of antibody they produce (e.g., from IgM to IgG, IgA, or IgE), while retaining the same antigen specificity.
- This process diversifies the effector functions of antibodies but does not directly describe the *improvement in antigen binding affinity* or specificity.
*T cell negative selection*
- **T cell negative selection** is a critical process in the thymus where T cells that react too strongly to self-antigens are eliminated or inactivated to prevent autoimmunity.
- This process is fundamental for establishing central tolerance in T cells and is separate from the B cell-mediated improvement in antibody specificity described.
*T cell positive selection*
- **T cell positive selection** also occurs in the thymus, ensuring that only T cells capable of recognizing self-MHC molecules survive and mature.
- This process is essential for T cell function (MHC restriction) but is distinct from the described mechanism of B cell antibody refinement.
Coagulase-negative staphylococci US Medical PG Question 10: A graph shows four different distribution curves labeled A, B, C, and D. Identify the correct sequence of distribution types shown in the graph.
- A. Skewed with outliers, positively skewed, negatively skewed, normal
- B. Normal, positively skewed, negatively skewed, normal with outliers (Correct Answer)
- C. Normal, negatively skewed, positively skewed, skewed with outliers
- D. Normal, negatively skewed, positively skewed, normal with outliers
Coagulase-negative staphylococci Explanation: This question tests the ability to identify different types of **statistical distributions** commonly encountered in biostatistics.
***Correct Answer: Normal, positively skewed, negatively skewed, normal with outliers***
This represents the correct sequence of four fundamental distribution patterns:
**Normal Distribution (A):**
- Symmetrical, bell-shaped curve
- Mean = Median = Mode
- Most data points cluster around the center
- Standard distribution pattern in many biological variables (e.g., height, blood pressure in populations)
**Positively Skewed Distribution (B):**
- Tail extends toward the **right (positive direction)**
- Mean > Median > Mode
- Common in data like: disease incubation periods, hospital length of stay, income distributions
- Clinical example: Most patients recover quickly, but few have prolonged illness
**Negatively Skewed Distribution (C):**
- Tail extends toward the **left (negative direction)**
- Mode > Median > Mean
- Less common in medical data
- Example: Age at death in developed countries (most die at older ages, few die young)
**Normal Distribution with Outliers (D):**
- Generally symmetrical bell shape
- Contains extreme values (outliers) that deviate significantly from the pattern
- Important to identify as outliers can affect statistical analysis
- Clinical example: Most patients respond normally to treatment, but few show extreme responses
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*Why Other Options Are Incorrect:*
The other options present incorrect sequences of the distribution types. The key is recognizing:
- The direction of skewness (tail direction)
- The difference between basic skewed distributions and those with outliers
- The symmetry of normal distributions
**Clinical Relevance:** Understanding distribution types is essential for:
- Choosing appropriate statistical tests (parametric vs non-parametric)
- Identifying data patterns in epidemiological studies
- Recognizing when data transformation is needed
- Detecting outliers that may represent errors or special cases
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