Viridans group streptococci US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Viridans group streptococci. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Viridans group streptococci US Medical PG Question 1: A 34-year-old man comes to the physician because of fatigue and shortness of breath with moderate exertion for the past 2 months. Over the past 10 days, he has had low-grade fevers and night sweats. He has no history of serious illness except for a bicuspid aortic valve diagnosed 5 years ago. He has smoked one pack of cigarettes daily for 10 years and drinks 3–5 beers on social occasions. He does not use illicit drugs. The patient takes no medications. He appears weak. His temperature is 37.7°C (99.9°F), pulse is 70/min, and blood pressure is 128/64 mm Hg. The lungs are clear to auscultation. A grade 2/6 systolic murmur is heard best at the right sternal border and second intercostal space. There are several hemorrhages underneath his fingernails on both hands and multiple tender, red nodules on his fingers. Which of the following is the most likely causal organism?
- A. Candida albicans
- B. Streptococcus sanguinis (Correct Answer)
- C. Streptococcus pneumoniae
- D. Staphylococcus epidermidis
Viridans group streptococci Explanation: ***Streptococcus sanguinis***
- The patient's history of **bicuspid aortic valve** represents a predisposing cardiac lesion for **infective endocarditis**.
- **S. sanguinis** is part of the **viridans group streptococci**, common inhabitants of the oral flora, and is a frequent cause of subacute bacterial endocarditis, especially in individuals with damaged heart valves.
- The clinical presentation of **splinter hemorrhages** and **Osler nodes** (tender nodules on fingers) along with low-grade fever and constitutional symptoms is classic for subacute bacterial endocarditis.
*Candida albicans*
- While *Candida* can cause endocarditis, it is typically seen in specific risk groups such as **intravenous drug users**, immunocompromised patients, or after prosthetic valve surgery, none of which apply here.
- Fungal endocarditis often presents with larger **vegetations** and a more subacute course, but bacterial causes (especially viridans streptococci) are far more common in this clinical setting.
*Streptococcus pneumoniae*
- *S. pneumoniae* (pneumococcus) is a known cause of **pneumonia**, **meningitis**, and **otitis media**, but it is an uncommon cause of endocarditis.
- Pneumococcal endocarditis, when it occurs, typically presents with a more fulminant course and may be associated with other sites of pneumococcal infection.
*Staphylococcus epidermidis*
- *S. epidermidis* is a common cause of **prosthetic valve endocarditis** and infections related to foreign bodies or catheters.
- Given the patient's **native valve** issue and absence of prosthetic material or recent invasive procedures, it is less likely than **viridans streptococci**.
Viridans group streptococci US Medical PG Question 2: A 23-year-old woman presents with a painful lesion in her mouth. She denies tooth pain, bleeding from the gums, nausea, vomiting, diarrhea, or previous episodes similar to this in the past. She states that her last normal menstrual period was 12 days ago, and she has not been sexually active since starting medical school 2 years ago. On physical examination, the patient has good dentition with no signs of infection with the exception of a solitary ulcerated lesion on the oral mucosa. The nonvesicular lesion has a clean gray-white base and is surrounded by erythema. Which of the following is correct concerning the most likely etiology of the oral lesion in this patient?
- A. This lesion is non-contagious but will most likely recur. (Correct Answer)
- B. This lesion is associated with an autoimmune disease characterized by a sensitivity to gluten.
- C. This lesion may progress to squamous cell carcinoma.
- D. This lesion is due to a fungal infection and may mean you're immunocompromised.
- E. This lesion is highly contagious and is due to reactivation of a dormant virus.
Viridans group streptococci Explanation: ***This lesion is non-contagious but will most likely recur.***
- The description of a **solitary, non-vesicular ulcerated lesion** with a **gray-white base** and surrounding **erythema** in the oral mucosa of a 23-year-old woman is highly characteristic of an **aphthous ulcer** (canker sore).
- Aphthous ulcers are **non-contagious** and are well-known for their **recurrent nature**, often appearing periodically throughout a person's life.
*This lesion is associated with an autoimmune disease characterized by a sensitivity to gluten.*
- While **celiac disease** (an autoimmune condition triggered by gluten) can be associated with recurrent aphthous ulcers, it is not the **most likely direct etiology** of the lesion itself, especially without other gastrointestinal symptoms or a history of celiac disease.
- The primary cause of aphthous ulcers in this context is typically unknown or related to minor trauma, stress, or nutritional deficiencies.
*This lesion may progress to squamous cell carcinoma.*
- Aphthous ulcers are **benign** and do not carry a risk of malignant transformation into **squamous cell carcinoma**.
- Risk factors for squamous cell carcinoma of the oral cavity include prolonged tobacco and alcohol use, and chronic irritation from ill-fitting dentures.
*This lesion is due to a fungal infection and may mean you're immunocompromised.*
- A fungal infection like **oral candidiasis** (thrush) typically presents as **white patches** that can be scraped off, often associated with immunocompromise or antibiotic use.
- The description of a well-demarcated ulcer with a gray-white base surrounded by erythema does not fit the typical presentation of a fungal infection.
*This lesion is highly contagious and is due to reactivation of a dormant virus.*
- Lesions caused by a dormant virus, such as **herpes simplex virus (HSV)**, typically present as **vesicles** that rupture to form ulcers, often appearing in clusters (cold sores).
- Aphthous ulcers are distinctly **non-viral** in origin and **not contagious**, differentiating them from herpetic lesions.
Viridans group streptococci US Medical PG Question 3: 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
Viridans group streptococci 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.
Viridans group streptococci US Medical PG Question 4: 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
Viridans group streptococci 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.
Viridans group streptococci US Medical PG Question 5: A 59-year-old man presents to the emergency room for a fever that has persisted for over 4 days. In addition, he has been experiencing weakness and malaise. His past medical history is significant for a bicuspid aortic valve that was replaced 2 years ago. Physical exam reveals nailbed splinter hemorrhages, tender nodules on his fingers, and retinal hemorrhages. An echocardiogram shows aortic valve vegetations and culture reveals a gram-positive alpha-hemolytic organism that grows as cocci in chains. The organism is then exposed to optochin and found to be resistant. Finally, they are seen to ferment sorbitol. The most likely cause of this patient's symptoms is associated with which of the following?
- A. Pneumonia
- B. Colon cancer
- C. IV drug use
- D. Dental procedures (Correct Answer)
- E. Sexual activity
Viridans group streptococci Explanation: ***Dental procedures***
- The patient's presentation with **infective endocarditis** (fever, weakness, malaise, splinter hemorrhages, Osler nodes [tender nodules], Roth spots [retinal hemorrhages], and valve vegetations) is caused by a gram-positive alpha-hemolytic, optochin-resistant, sorbitol-fermenting coccus in chains.
- These microbiologic characteristics identify **viridans group streptococci** (such as *Streptococcus mutans* or *S. sanguinis*), which are **normal oral flora**.
- The patient's **prosthetic aortic valve** makes him particularly susceptible to endocarditis, and **dental procedures** can introduce these oral bacteria into the bloodstream, leading to bacteremia and valve seeding.
- This is why patients with prosthetic valves require **antibiotic prophylaxis** before dental procedures.
*Pneumonia*
- While pneumonia can cause fever and malaise, it doesn't lead to the characteristic peripheral stigmata of **endocarditis** like splinter hemorrhages, Osler nodes, or Roth spots, nor valve vegetations.
- The organism isolated (alpha-hemolytic, optochin-resistant viridans streptococci) is normal oral flora, not a typical cause of pneumonia.
*Colon cancer*
- **Colon cancer** is strongly associated with endocarditis caused by ***Streptococcus gallolyticus* (formerly *S. bovis* biotype I)**.
- However, the microbiologic description in this case (particularly optochin resistance and sorbitol fermentation) identifies **viridans streptococci**, not *S. gallolyticus*.
- If this were *S. gallolyticus* endocarditis, colonoscopy would be indicated to screen for colorectal malignancy.
*IV drug use*
- **IV drug use** is a major risk factor for **right-sided endocarditis**, particularly involving the **tricuspid valve**, typically caused by ***Staphylococcus aureus***.
- This patient has **left-sided** (aortic) **prosthetic valve endocarditis** caused by viridans streptococci, which is not the typical pattern for IV drug use.
*Sexual activity*
- **Sexual activity** is not a risk factor for **infective endocarditis** caused by viridans streptococci.
- While certain sexually transmitted pathogens can rarely cause systemic complications, they do not predispose to endocarditis with oral flora organisms.
Viridans group streptococci US Medical PG Question 6: A 42-year-old woman presents to a medical office with complaints of fatigue, weight loss, and low-grade fever for 1 week. She noticed bleeding spots on her feet this morning. The past medical history is significant for a recent dental appointment. She is a non-smoker and does not drink alcohol. She does not currently take any medications. On examination, the vital signs include temperature 37.8°C (100.0°F), blood pressure 138/90 mm Hg, respirations 21/min, and pulse 87/min. Cardiac auscultation reveals a pansystolic murmur in the mitral area with radiation to the left axilla. Laboratory studies show hemoglobin levels of 17.2 g/dL, erythrocyte sedimentation rate (ESR) of 25 mm/h, and a white blood cell (WBC) count of 12,000 cells/mm3. An echocardiogram reveals valvular vegetations on the mitral valve with mild regurgitation. Blood samples are sent for bacterial culture. Empiric antibiotic therapy is initiated with ceftriaxone and vancomycin. The blood cultures most likely will yield the growth of which of the following organisms?
- A. Actinomyces israelii
- B. Coxiella burnetii
- C. Group B Streptococcus
- D. Staphylococcus aureus
- E. Streptococcus viridans (Correct Answer)
Viridans group streptococci Explanation: ***Streptococcus viridans***
- The patient's recent **dental appointment**, the presence of an **oral organism**, and symptoms consistent with **subacute bacterial endocarditis** (fatigue, fever, weight loss, petechiae) strongly suggest *S. viridans* as the causative agent.
- *Streptococcus viridans* species are common inhabitants of the **oral flora** and are a leading cause of endocarditis following dental procedures, especially in individuals with pre-existing valvular abnormalities.
*Actinomyces israelii*
- While *Actinomyces israelii* is an oral commensal, it typically causes **actinomycosis**, a chronic, suppurative infection characterized by abscess formation and sinus tracts, often following dental procedures.
- It less commonly presents as acute or subacute endocarditis and would not typically cause the rapid progression of symptoms described.
*Coxiella burnetii*
- *Coxiella burnetii* causes **Q fever**, which can manifest as endocarditis, often associated with exposure to **farm animals** or their products.
- The patient's history lacks any such exposure, and the clinical presentation is more aligned with standard bacterial endocarditis from oral flora.
*Group B Streptococcus*
- **Group B Streptococcus** (*Streptococcus agalactiae*) is primarily known as a cause of **neonatal sepsis** and meningitis, and infections in immunocompromised adults or those with underlying conditions like diabetes.
- It is not typically associated with endocarditis following a dental procedure in an otherwise healthy adult.
*Staphylococcus aureus*
- **Staphylococcus aureus** can cause endocarditis, particularly in **intravenous drug users** or patients with prosthetic valves/indwelling lines, and often presents as a more **acute and aggressive disease**.
- While possible, the association with a recent dental procedure and the subacute course makes *Streptococcus viridans* a more likely culprit in this specific scenario.
Viridans group streptococci US Medical PG Question 7: A 5-year-old boy is brought into your office by his mother. His father recently passed away, and his mother states she just lost her job. She has been unable to buy food regularly, and they have had to eat boiled and preserved vegetables. His mother denies that the boy has any prior medical conditions, but the patient states that his gums bleed when he brushes his teeth. On exam, the patient's vital signs are normal, but he appears malnourished. There is gum hypertrophy present on exam along with small, curled hairs over his head. CBC is significant for a Hgb of 9.5 g/dL with an MCV of 85. PT, aPTT, and bleeding time are all normal. What is the most likely cause?
- A. Vitamin C deficiency (Correct Answer)
- B. Vitamin K deficiency
- C. Vitamin B3 deficiency
- D. Vitamin B12 deficiency
- E. Iron deficiency
Viridans group streptococci Explanation: ***Vitamin C deficiency***
- The patient's history of **malnutrition**, consumption of only **boiled and preserved vegetables** (which deplete vitamin C), **bleeding gums**, and **curled hairs** ("corkscrew hairs") are classic signs of scurvy, caused by vitamin C deficiency.
- **Vitamin C** is essential for **collagen synthesis** and *antioxidant function*, and its deficiency leads to impaired wound healing, fragile capillaries, and anemia.
*Vitamin K deficiency*
- This deficiency primarily affects **blood clotting**, leading to increased PT/aPTT, but the patient's **PT, aPTT, and bleeding time are all normal**.
- While bleeding gums can occur, it's not typically associated with **curled hairs** or caused by a diet of boiled vegetables in this manner.
*Vitamin B3 deficiency*
- Also known as **niacin deficiency**, it causes pellagra, characterized by the "3 Ds": **dermatitis**, **diarrhea**, and **dementia**.
- These symptoms are not present in the patient, and bleeding gums or curled hairs are not typical manifestations of niacin deficiency.
*Vitamin B12 deficiency*
- This deficiency usually results in **macrocytic anemia** (high MCV), **neurological symptoms**, and **glossitis** (sore, smooth tongue).
- The patient has a **normal MCV** (85) and none of the characteristic neurological or oral findings of B12 deficiency.
*Iron deficiency*
- Iron deficiency leads to **microcytic anemia** (low MCV) and symptoms like **fatigue**, **pallor**, and **pica**.
- The patient has **normocytic anemia** (MCV 85), and while anemia is present, the specific constellation of bleeding gums and curled hairs is not characteristic of iron deficiency.
Viridans group streptococci US Medical PG Question 8: A 63-year-old man with aortic valve disease is admitted to the hospital for a 3-week history of progressively worsening fatigue, fever, and night sweats. He does not smoke, drink alcohol, or use illicit drugs. Temperature is 38.2°C (100.8°F). Physical examination shows a systolic murmur and tender, erythematous nodules on the finger pads. Blood cultures show alpha-hemolytic, gram-positive cocci that are catalase-negative and optochin-resistant. Which of the following is the most likely causal organism?
- A. Streptococcus pneumoniae
- B. Staphylococcus epidermidis
- C. Viridans streptococci (Correct Answer)
- D. Streptococcus pyogenes
- E. Streptococcus gallolyticus
Viridans group streptococci Explanation: ***Viridans streptococci***
- The patient's presentation with **subacute onset** of fever, fatigue, cardiac murmur, and **Osler nodes** (tender finger nodules) points to **infective endocarditis**. The micro-organism is described as **alpha-hemolytic**, **catalase-negative**, and **optochin-resistant**, which are characteristic features of **Viridans streptococci**.
- **Viridans streptococci** are a common cause of **subacute bacterial endocarditis**, especially in patients with pre-existing valvular disease like the **aortic valve disease** mentioned.
*Streptococcus pneumoniae*
- While **Streptococcus pneumoniae** is also **alpha-hemolytic** and **catalase-negative**, it is typically **optochin-sensitive** and a common cause of **pneumonia** and **meningitis**, not usually subacute endocarditis from oral flora.
- Endocarditis caused by *S. pneumoniae* is rare and usually associated with a more fulminant course.
*Staphylococcus epidermidis*
- **Staphylococcus epidermidis** is a **coagulase-negative staphylococcus** that is a common cause of **prosthetic valve endocarditis** and is **catalase-positive**, unlike the organism described here.
- It is not typically alpha-hemolytic.
*Streptococcus pyogenes*
- **Streptococcus pyogenes** is **beta-hemolytic** and **catalase-negative**, and typically causes **pharyngitis** and **skin infections**, or sometimes **acute endocarditis**.
- It does not fit the description of an **alpha-hemolytic**, **optochin-resistant** organism.
*Streptococcus gallolyticus*
- **Streptococcus gallolyticus** (formerly *Streptococcus bovis*) is associated with **bacteremia** and **endocarditis**, particularly in patients with **gastrointestinal malignancies**.
- While it is **alpha-hemolytic** and **catalase-negative**, it is typically differentiated by its growth in **bile esculin** and is not primarily defined by optochin resistance characteristic of Viridans group.
Viridans group streptococci US Medical PG Question 9: A 45-year-old man comes to the physician because of a 1-month history of fever and poor appetite. Five weeks ago, he underwent molar extraction for dental caries. His temperature is 38°C (100.4°F). Cardiac examination shows a grade 2/6 holosystolic murmur heard best at the apex. A blood culture shows gram-positive, catalase-negative cocci. Transesophageal echocardiography shows a small vegetation on the mitral valve with mild regurgitation. The causal organism most likely has which of the following characteristics?
- A. Production of dextrans (Correct Answer)
- B. Production of CAMP factor
- C. Conversion of fibrinogen to fibrin
- D. Formation of germ tubes at body temperature
- E. Replication in host macrophages
Viridans group streptococci Explanation: **Production of dextrans**
- The clinical picture of **fever**, **poor appetite**, a **holosystolic murmur**, and **mitral valve vegetation** following a dental procedure (molar extraction) strongly points to **infective endocarditis** caused by **Viridans streptococci**.
- **Viridans streptococci**, commonly found in the oral cavity, produce **dextrans**, which allow them to adhere to damaged heart valves and fibrin-platelet aggregates, initiating vegetation formation.
*Production of CAMP factor*
- **CAMP factor** is a characteristic of **Group B Streptococcus (Streptococcus agalactiae)**, which primarily causes infections in neonates and immunocompromised adults, not typically infective endocarditis post-dental procedure.
- *Streptococcus agalactiae* is also catalase-negative and gram-positive but is rarely associated with endocarditis arising from oral flora.
*Conversion of fibrinogen to fibrin*
- The ability to convert **fibrinogen to fibrin** is characteristic of **coagulase-positive organisms**, such as *Staphylococcus aureus*, which is a catalase-positive organism.
- The blood culture in this case specifically states **catalase-negative cocci**, ruling out *Staphylococcus aureus* as the causative agent.
*Formation of germ tubes at body temperature*
- **Germ tube formation** at body temperature is a distinguishing characteristic of *Candida albicans*, a **fungus**, not a gram-positive, catalase-negative coccus.
- While *Candida* can cause endocarditis, the microbiological findings described do not align with a fungal infection.
*Replication in host macrophages*
- **Intracellular replication in host macrophages** is characteristic of certain bacteria like *Mycobacterium tuberculosis*, *Listeria monocytogenes*, or *Salmonella typhi*, which typically cause systemic infections
- This characteristic is not associated with the gram-positive, catalase-negative cocci responsible for subacute bacterial endocarditis following dental procedures.
Viridans group streptococci US Medical PG Question 10: 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
Viridans group streptococci 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.
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