A 25-year-old man with no significant past medical history is brought in by ambulance after a witnessed seizure at home. On physical exam, temperature is 102.3 deg F (39.1 deg C), blood pressure is 90/62 mmHg, pulse is 118/min, and respirations are 25/min. He is unable to touch his chin to his chest and spontaneously flexes his hips with passive neck flexion. Appropriate empiric treatment is begun. CT head is unremarkable, and a lumbar puncture sample is obtained. Gram stain of the cerebrospinal fluid (CSF) reveals gram-positive diplococci. Which of the following would you expect to see on CSF studies?
Q62
A 22-year-old woman at 30 weeks gestation presents to the obstetrician with the sudden onset of fever, headache, anorexia, fatigue, and malaise. She mentioned that she had eaten ice cream 3 days ago. Blood cultures show gram-positive rods that are catalase-positive and display distinctive tumbling motility in liquid medium. What is the most likely diagnosis?
Q63
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?
Q64
A 30-year-old woman comes to the emergency department because of fever, watery diarrhea, and abdominal cramping for the past 24 hours. She recently went to an international food fair. Her temperature is 39°C (102.2°F). Physical examination shows increased bowel sounds. Stool cultures grow gram-positive, spore-forming, anaerobic rods that produce alpha toxin. The responsible organism also causes which of the following physical examination findings?
Q65
A previously healthy 3-month-old girl is brought to the physician because of fever, irritability, and rash for 3 days. The rash started around the mouth before spreading to the trunk and extremities. Her temperature is 38.6°C (101.5°F). Examination shows a diffuse erythematous rash with flaccid bullae on the neck, flexural creases, and buttocks. Gentle pressure across the trunk with a gloved finger creates a blister. Oropharyngeal examination shows no abnormalities. Which of the following is the most likely underlying mechanism of these skin findings?
Q66
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?
Q67
A 71-year-old man comes to the physician because of a 2-week history of fatigue and a cough productive of a blood-tinged phlegm. Over the past month, he has had a 5.0-kg (11-lb) weight loss. He has hypertension and type 2 diabetes mellitus. Eight months ago, he underwent a kidney transplantation. The patient does not smoke. His current medications include lisinopril, insulin, prednisone, and mycophenolate mofetil. His temperature is 38.9°C (102.1°F), pulse is 88/min, and blood pressure is 152/92 mm Hg. Rhonchi are heard at the right lower lobe of the lung on auscultation. There is a small ulceration on the left forearm. An x-ray of the chest shows a right lung mass with lobar consolidation. Antibiotic therapy with levofloxacin is started. Three days later, the patient has a seizure and difficulty coordinating movements with his left hand. An MRI of the brain shows an intraparenchymal lesion with peripheral ring enhancement. Bronchoscopy with bronchoalveolar lavage yields weakly acid-fast, gram-positive bacteria with branching, filamentous shapes. Which of the following is the most appropriate initial pharmacotherapy?
Q68
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?
Q69
A 2980-g (6.6-lb) female newborn is brought to the emergency department by her mother because of worsening lethargy. The newborn was delivered at home 10 hours ago. The mother has had no prenatal care. The newborn's temperature is 39.7°C (103.5°F). Physical examination shows scleral icterus. Her leukocyte count is 36,000/mm3 (85% segmented neutrophils). An organism is isolated from the blood. When grown together on sheep agar, the isolated organism enlarges the area of clear hemolysis formed by Staphylococcus aureus. Which of the following is the most likely causal organism?
Q70
A 6-year-old girl is brought to the physician by her father because of a 3-day history of sore throat, abdominal pain, nausea, vomiting, and high fever. She has been taking acetaminophen for the fever. Physical examination shows cervical lymphadenopathy, pharyngeal erythema, and a bright red tongue. Examination of the skin shows a generalized erythematous rash with a rough surface that spares the area around the mouth. Which of the following is the most likely underlying mechanism of this patient's rash?
Gram-positive US Medical PG Practice Questions and MCQs
Question 61: A 25-year-old man with no significant past medical history is brought in by ambulance after a witnessed seizure at home. On physical exam, temperature is 102.3 deg F (39.1 deg C), blood pressure is 90/62 mmHg, pulse is 118/min, and respirations are 25/min. He is unable to touch his chin to his chest and spontaneously flexes his hips with passive neck flexion. Appropriate empiric treatment is begun. CT head is unremarkable, and a lumbar puncture sample is obtained. Gram stain of the cerebrospinal fluid (CSF) reveals gram-positive diplococci. Which of the following would you expect to see on CSF studies?
A. Elevated opening pressure, elevated protein, normal glucose
B. Normal opening pressure, normal protein, normal glucose
C. Elevated opening pressure, elevated protein, elevated glucose
D. Normal opening pressure, elevated protein, normal glucose
E. Elevated opening pressure, elevated protein, low glucose (Correct Answer)
Explanation: ***Elevated opening pressure, elevated protein, low glucose***
- **Bacterial meningitis** typically presents with an **elevated opening pressure** due to inflammation and increased intracranial pressure.
- The inflammatory response leads to increased vascular permeability, causing **elevated protein** in the CSF and **low glucose** due to bacterial consumption.
*Elevated opening pressure, elevated protein, normal glucose*
- While **elevated opening pressure** and **elevated protein** can be seen in inflammatory conditions, **normal CSF glucose** is more characteristic of viral meningitis or non-infectious inflammatory conditions, not bacterial.
- In bacterial meningitis, bacteria metabolize glucose, leading to a **decreased CSF glucose level**.
*Normal opening pressure, normal protein, normal glucose*
- This profile is typical for a **normal CSF study** or certain chronic neurological conditions, not acute bacterial meningitis.
- The patient's severe symptoms, including fever, seizure, and meningismus, contradict a normal CSF profile.
*Elevated opening pressure, elevated protein, elevated glucose*
- While **elevated opening pressure** and **elevated protein** can occur, **elevated CSF glucose** is inconsistent with any common form of meningitis.
- An elevated CSF glucose is rare and would suggest systemic hyperglycemia without impaired glucose transport into the CSF.
*Normal opening pressure, elevated protein, normal glucose*
- A **normal opening pressure** would be unusual in a patient with bacterial meningitis, given the inflammation and potential for increased intracranial pressure.
- While **elevated protein** and **normal glucose** can occur in certain conditions (e.g., Guillain-Barré syndrome), it does not fit the clinical picture of acute bacterial meningitis with fever and meningismus.
Question 62: A 22-year-old woman at 30 weeks gestation presents to the obstetrician with the sudden onset of fever, headache, anorexia, fatigue, and malaise. She mentioned that she had eaten ice cream 3 days ago. Blood cultures show gram-positive rods that are catalase-positive and display distinctive tumbling motility in liquid medium. What is the most likely diagnosis?
A. Tularaemia
B. Brucellosis
C. Legionnaires' disease
D. Influenza
E. Listeriosis (Correct Answer)
Explanation: ***Listeriosis***
- The patient's symptoms (fever, headache, anorexia, fatigue, malaise), pregnancy status (30 weeks gestation), recent consumption of **ice cream** (a potential source of **Listeria monocytogenes**), and blood culture findings of **gram-positive rods** that are **catalase-positive** with **tumbling motility** are all classic indicators of listeriosis.
- **Listeria monocytogenes** is known to cause severe illness in pregnant women and has a characteristic **tumbling motility** due to flagella and can grow at refrigerator temperatures, making contaminated dairy products a common source.
*Tularaemia*
- Tularaemia is caused by **Francisella tularensis**, which is a **gram-negative coccobacillus**, not a gram-positive rod, and typically transmitted through contact with infected animals or insect bites.
- It usually presents with a papule that ulcerates, lymphadenopathy, and systemic symptoms, which are not described here.
*Brucellosis*
- Brucellosis is caused by bacteria of the genus **Brucella**, which are **gram-negative coccobacillary rods**, not gram-positive, and typically transmitted from unpasteurized dairy products or contact with infected animals.
- While it can present with fever and malaise, the gram stain and motility characteristics in the blood culture rule it out.
*Legionnaires' disease*
- Legionnaires' disease is caused by **Legionella pneumophila**, a **gram-negative rod** that is difficult to culture and often diagnosed via urine antigen test for Legionella serogroup 1.
- It primarily causes pneumonia and does not typically present with the described blood culture findings.
*Influenza*
- Influenza is a **viral infection** and would not show **gram-positive rods** or any bacterial growth on blood cultures.
- While it presents with fever, headache, and fatigue, the microbiological findings are inconsistent with a viral etiology.
Question 63: 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
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.
Question 64: A 30-year-old woman comes to the emergency department because of fever, watery diarrhea, and abdominal cramping for the past 24 hours. She recently went to an international food fair. Her temperature is 39°C (102.2°F). Physical examination shows increased bowel sounds. Stool cultures grow gram-positive, spore-forming, anaerobic rods that produce alpha toxin. The responsible organism also causes which of the following physical examination findings?
A. Diffuse, flaccid bullae
B. Subcutaneous crepitus (Correct Answer)
C. Facial paralysis
D. Rose spots
E. Petechial rash
Explanation: ***Subcutaneous crepitus***
- The description of gram-positive, spore-forming, anaerobic rods producing alpha toxin is characteristic of *Clostridium perfringens*.
- This organism causes **two main clinical syndromes**: (1) **food poisoning** with diarrhea (as in this patient), and (2) **gas gangrene** (clostridial myonecrosis).
- **Gas gangrene** is characterized by muscle necrosis, gas production in tissues (leading to **crepitus** on palpation), and rapid tissue destruction.
*Diffuse, flaccid bullae*
- This finding is more commonly associated with **staphylococcal scalded skin syndrome (SSSS)** caused by *Staphylococcus aureus* exfoliative toxins.
- *Clostridium perfringens* infections typically lead to **gas formation** and tissue necrosis rather than superficial bullae.
*Facial paralysis*
- **Facial paralysis** is characteristic of *Clostridium botulinum* (botulism), which produces neurotoxins that block acetylcholine release.
- *Clostridium perfringens* does not produce neurotoxins that cause paralysis; its pathogenicity is due to **alpha toxin** (phospholipase C) causing tissue destruction.
*Rose spots*
- **Rose spots** are characteristic of **typhoid fever**, caused by *Salmonella Typhi*.
- They are faint, salmon-colored maculopapular lesions on the trunk that blanch with pressure.
*Petechial rash*
- A **petechial rash** is often seen in conditions like **meningococcemia** (*Neisseria meningitidis*), **Rocky Mountain spotted fever**, or bacterial **endocarditis** due to vascular damage.
- While *Clostridium perfringens* can cause severe sepsis, a petechial rash is not its classic presentation.
Question 65: A previously healthy 3-month-old girl is brought to the physician because of fever, irritability, and rash for 3 days. The rash started around the mouth before spreading to the trunk and extremities. Her temperature is 38.6°C (101.5°F). Examination shows a diffuse erythematous rash with flaccid bullae on the neck, flexural creases, and buttocks. Gentle pressure across the trunk with a gloved finger creates a blister. Oropharyngeal examination shows no abnormalities. Which of the following is the most likely underlying mechanism of these skin findings?
A. Bacterial invasion of the epidermis
B. Toxin-induced cleavage of desmoglein (Correct Answer)
C. Autoantibody deposition in stratum spinosum
D. Bacterial production of erythrogenic toxin
E. Autoantibody binding of hemidesmosomes
Explanation: **Toxin-induced cleavage of desmoglein**
- The clinical presentation of **fever**, **irritability**, **diffuse erythematous rash with flaccid bullae**, and a **positive Nikolsky sign** (blister formation with gentle pressure) in an infant is characteristic of **staphylococcal scalded skin syndrome (SSSS)**.
- SSSS is caused by **exfoliative exotoxins (ETA and ETB)** produced by *Staphylococcus aureus* that target and cleave **desmoglein 1**, a component of **desmosomes** responsible for cell-to-cell adhesion in the stratum granulosum of the epidermis.
*Bacterial invasion of the epidermis*
- While *Staphylococcus aureus* is the causative agent, **bacterial invasion of the epidermis itself does not occur in SSSS**. The damage is mediated solely by the circulating toxins.
- Conditions like **impetigo** involve direct bacterial invasion, but they typically present with crusted lesions rather than widespread bullae and systemic symptoms characteristic of SSSS.
*Autoantibody deposition in stratum spinosum*
- This mechanism is characteristic of **pemphigus vulgaris**, a severe autoimmune blistering disease.
- Pemphigus vulgaris typically affects older individuals and presents with painful, flaccid bullae on the skin and mucous membranes, but it is not seen in healthy infants in this context.
*Bacterial production of erythrogenic toxin*
- **Erythrogenic toxins** (also known as streptococcal pyrogenic exotoxins) are produced by *Streptococcus pyogenes* and are responsible for the rash of **scarlet fever**.
- The rash of scarlet fever is a diffuse erythema with a sandpaper-like texture and Pastia's lines, not widespread flaccid bullae and Nikolsky sign.
*Autoantibody binding of hemidesmosomes*
- This mechanism describes **bullous pemphigoid**, another autoimmune blistering disease, where autoantibodies target components of **hemidesmosomes** (e.g., BP180 and BP230) at the dermal-epidermal junction.
- Bullous pemphigoid typically presents with tense bullae in older adults and is not consistent with the acute presentation in an infant seen here.
Question 66: 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
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.
Question 67: A 71-year-old man comes to the physician because of a 2-week history of fatigue and a cough productive of a blood-tinged phlegm. Over the past month, he has had a 5.0-kg (11-lb) weight loss. He has hypertension and type 2 diabetes mellitus. Eight months ago, he underwent a kidney transplantation. The patient does not smoke. His current medications include lisinopril, insulin, prednisone, and mycophenolate mofetil. His temperature is 38.9°C (102.1°F), pulse is 88/min, and blood pressure is 152/92 mm Hg. Rhonchi are heard at the right lower lobe of the lung on auscultation. There is a small ulceration on the left forearm. An x-ray of the chest shows a right lung mass with lobar consolidation. Antibiotic therapy with levofloxacin is started. Three days later, the patient has a seizure and difficulty coordinating movements with his left hand. An MRI of the brain shows an intraparenchymal lesion with peripheral ring enhancement. Bronchoscopy with bronchoalveolar lavage yields weakly acid-fast, gram-positive bacteria with branching, filamentous shapes. Which of the following is the most appropriate initial pharmacotherapy?
A. Trimethoprim/sulfamethoxazole (Correct Answer)
B. Rifampin, isoniazid, pyrazinamide, and ethambutol
C. Piperacillin/tazobactam
D. Vancomycin
E. Erythromycin
Explanation: **Trimethoprim/sulfamethoxazole**
- The patient's presentation with **fever**, **cough with blood-tinged phlegm**, **weight loss**, **pulmonary mass with lobar consolidation**, **brain lesion with ring enhancement**, and **weakly acid-fast, gram-positive branching filamentous bacteria** is highly suggestive of **Nocardia infection**.
- **Trimethoprim/sulfamethoxazole (TMP/SMX)** is the **first-line treatment** for Nocardia infections, particularly in immunocompromised patients like this renal transplant recipient on immunosuppressants (prednisone, mycophenolate mofetil).
*Rifampin, isoniazid, pyrazinamide, and ethambutol*
- This drug combination is the standard therapy for **tuberculosis (TB)**, which is caused by *Mycobacterium tuberculosis*.
- While TB can present with lung and brain lesions, the identification of **weakly acid-fast, *gram-positive branching filamentous* bacteria** is characteristic of Nocardia, not Mycobacterium.
*Piperacillin/tazobactam*
- This is a broad-spectrum antibiotic effective against many **gram-positive**, **gram-negative**, and **anaerobic bacteria**.
- It is not typically used for Nocardia infections and would not be the most appropriate initial therapy given the specific microbiological findings.
*Vancomycin*
- **Vancomycin** is an antibiotic primarily used for **gram-positive infections**, especially those caused by **methicillin-resistant *Staphylococcus aureus* (MRSA)** or **Clostridioides difficile**.
- It is **not effective** against Nocardia species.
*Erythromycin*
- **Erythromycin** is a macrolide antibiotic effective against certain **gram-positive bacteria**, atypical pneumonia pathogens (e.g., *Mycoplasma*, *Legionella*), and some sexually transmitted infections.
- It is **not the drug of choice** for Nocardia infections.
Question 68: 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)
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.
Question 69: A 2980-g (6.6-lb) female newborn is brought to the emergency department by her mother because of worsening lethargy. The newborn was delivered at home 10 hours ago. The mother has had no prenatal care. The newborn's temperature is 39.7°C (103.5°F). Physical examination shows scleral icterus. Her leukocyte count is 36,000/mm3 (85% segmented neutrophils). An organism is isolated from the blood. When grown together on sheep agar, the isolated organism enlarges the area of clear hemolysis formed by Staphylococcus aureus. Which of the following is the most likely causal organism?
A. Pseudomonas aeruginosa
B. Streptococcus agalactiae (Correct Answer)
C. Listeria monocytogenes
D. Streptococcus pyogenes
E. Streptococcus pneumoniae
Explanation: ***Streptococcus agalactiae***
- The description of a bacterial organism "enlarging the area of clear hemolysis formed by *Staphylococcus aureus*" on sheep agar refers to the **CAMP test**, a characteristic strongly associated with ***Streptococcus agalactiae*** (Group B Streptococcus, GBS).
- GBS is a leading cause of **neonatal sepsis and meningitis**, especially in newborns whose mothers received no prenatal care, as it can be transmitted vertically during birth.
*Pseudomonas aeruginosa*
- While *Pseudomonas aeruginosa* can cause severe infections, it is an **aerobic gram-negative rod** and would not typically exhibit the described CAMP test phenomenon.
- *Pseudomonas* infections in newborns are less common and often associated with **nosocomial outbreaks** or specific risk factors not primarily indicated here.
*Listeria monocytogenes*
- *Listeria monocytogenes* can cause **neonatal sepsis and meningitis** and is acquired transplacentally or intrapartum.
- However, it does not produce the characteristic **CAMP factor** that enhances hemolysis of *Staphylococcus aureus*.
*Streptococcus pyogenes*
- *Streptococcus pyogenes* (Group A Streptococcus) causes diseases like **strep throat** and **scarlet fever**, and rarely neonatal sepsis.
- It exhibits **beta-hemolysis** but typically tests **negative for the CAMP test**.
*Streptococcus pneumoniae*
- *Streptococcus pneumoniae* is a common cause of **pneumonia and meningitis** in infants and young children.
- It is an **alpha-hemolytic** organism and would not produce enhanced hemolysis in the CAMP test, which is characteristic of beta-hemolytic GBS.
Question 70: A 6-year-old girl is brought to the physician by her father because of a 3-day history of sore throat, abdominal pain, nausea, vomiting, and high fever. She has been taking acetaminophen for the fever. Physical examination shows cervical lymphadenopathy, pharyngeal erythema, and a bright red tongue. Examination of the skin shows a generalized erythematous rash with a rough surface that spares the area around the mouth. Which of the following is the most likely underlying mechanism of this patient's rash?
A. Bacterial invasion of the deep dermis
B. Anti-M protein antibody cross-reaction
C. Erythrogenic toxin-induced cytokine release (Correct Answer)
D. Paramyxovirus-induced cell damage
E. Subepithelial immune complex deposition
Explanation: ***Erythrogenic toxin-induced cytokine release***
- The clinical presentation, including **sore throat**, fever, **cervical lymphadenopathy**, **pharyngeal erythema**, a **bright red tongue (strawberry tongue)**, and a **generalized erythematous rash with a rough surface that spares the area around the mouth (scarlatiniform rash or circumoral pallor)**, is highly characteristic of **scarlet fever**.
- Scarlet fever is caused by strains of **Streptococcus pyogenes (Group A Streptococcus)** that produce **streptococcal pyrogenic exotoxins (SPEs)**, also known as **erythrogenic toxins**. These toxins act as superantigens, leading to widespread T-cell activation and massive cytokine release, which causes the characteristic rash.
*Bacterial invasion of the deep dermis*
- **Bacterial invasion of the deep dermis**, particularly by *Streptococcus pyogenes*, is characteristic of conditions like **erysipelas** or **cellulitis**, which cause localized, painful, and often rapidly spreading skin infections.
- These conditions typically do not present with a generalized, sandpapery rash, strawberry tongue, or systemic symptoms indicative of toxin-mediated illness like scarlet fever.
*Anti-M protein antibody cross-reaction*
- **Anti-M protein antibodies** are involved in the pathogenesis of **acute rheumatic fever (ARF)** and **post-streptococcal glomerulonephritis (PSGN)**, which are delayed sequelae of *Streptococcus pyogenes* infections.
- These antibodies cross-react with host tissues (e.g., heart, joints, brain in ARF; kidney in PSGN), leading to inflammation and damage, but they do not directly cause the acute scarlatiniform rash.
*Paramyxovirus-induced cell damage*
- **Paramyxoviruses** (e.g., measles, mumps, RSV) cause rashes, but they are typically **maculopapular** or vesicular, and the mechanism involves direct **viral replication and cell damage** in the skin, as well as immune responses to the virus.
- The rash of scarlet fever is distinct, being **erythematous and rough (sandpapery)**, and the extensive pharyngitis and strawberry tongue are not typical features of paramyxovirus infections.
*Subepithelial immune complex deposition*
- **Subepithelial immune complex deposition** is a characteristic feature of various **glomerulonephritides**, such as **post-streptococcal glomerulonephritis** or **lupus nephritis**.
- While immune complex deposition can cause dermatological manifestations in certain autoimmune diseases (e.g., vasculitis), it is not the mechanism for the acute rash seen in scarlet fever, which is directly toxin-mediated.
