A 10-year-old girl is brought to the emergency department because of a 2-day history of bloody diarrhea and abdominal pain. Four days ago, she visited a petting zoo with her family. Her temperature is 39.4°C (102.9°F). Abdominal examination shows tenderness to palpation of the right lower quadrant. Stool cultures at 42°C grow colonies that turn black after adding phenylenediamine. Which of the following best describes the most likely causal organism?
Q12
A 29-year-old man comes to the physician because of a 3-day history of a swollen right knee. Over the past several weeks, he has had similar episodes affecting the right knee and sometimes also the left elbow, in which the swelling lasted an average of 5 days. He has a history of a rash that subsided 2 months ago. He lives in Connecticut with his wife and works as a landscaper. His temperature is 37.8°C (100°F), pulse is 90/min, respirations are 12/min, and blood pressure is 110/75 mm Hg. Physical examination shows a tender and warm right knee; range of motion is limited by pain. The remainder of the examination shows no abnormalities. His hematocrit is 44%, leukocyte count is 10,300/mm3, and platelet count is 145,000/mm3. Serum electrolyte concentrations are within normal limits. Arthrocentesis is performed and the synovial fluid is cloudy. Gram stain is negative. Analysis of the synovial fluid shows a leukocyte count of 70,000/mm3 and 80% neutrophils. Serologic testing confirms the diagnosis. Which of the following is the most likely cause?
Q13
An immunology expert is explaining the functions of macrophages to biology students. He describes a hypothetical case scenario as follows: a potentially harmful gram-negative bacillus encounters a macrophage in the tissues. The Toll-like receptor (TLR) on the macrophage recognizes the bacterial lipopolysaccharide (LPS). The macrophage is activated by the binding of TLR with bacterial LPS and by interferon-γ (IFN-γ). Which of the following cytokines is most likely to be secreted by the activated macrophage?
Q14
An 8-year-old boy is brought to the emergency department because of a 4-day history of severe, left-sided ear pain and purulent discharge from his left ear. One week ago, he returned with his family from their annual summer vacation at a lakeside cabin, where he spent most of the time outdoors hiking and swimming. Examination shows tragal tenderness and a markedly edematous and erythematous external auditory canal. Audiometry shows conductive hearing loss of the left ear. Which of the following is the most likely cause of this patient's symptoms?
Q15
An 18-year-old college student seeks evaluation at an emergency department with complaints of fevers with chills, fatigue, diarrhea, and loss of appetite, which have lasted for 1 week. He says that his symptoms are progressively getting worse. He was taking over-the-counter acetaminophen, but it was ineffective. The past medical history is insignificant. His temperature is 38.8°C (101.9°F) and his blood pressure is 100/65 mm Hg. The physical examination is within normal limits, except that the patient appears ill. Eventually, a diagnosis of typhoid fever was established and he is started on appropriate antibiotics. Which of the following cellular components is most likely to be responsible for the toxic symptoms in this patient?
Q16
A stool sample was taken from a 19-year-old male who presented with profuse watery diarrhea. He recently returned from a trip to Central America. A microbiologist identified the causative agent as a gram-negative, oxidase-positive, comma-shaped bacteria that is able to grow well in a pH > 8. Which of the following is a mechanism of action of the toxin produced by this bacteria?
Q17
A 14-year-old boy presents with abdominal pain and diarrhea after returning from an East Asian vacation. Stool sample reveals the presence of red and white blood cells. Stool culture shows growth of immobile, non-lactose fermenting gram-negative rods. The attending physician explains to the medical students that the bacteria function by invading intestinal M-cells. The bacterium responsible for this patient's infection is:
Q18
A 62-year-old woman with type 2 diabetes mellitus is brought to the emergency department by her husband because of fever, chills, and purulent drainage from a foot ulcer for 2 days. Her hemoglobin A1c was 15.4% 16 weeks ago. Physical examination shows a 2-cm ulcer on the plantar surface of the left foot with foul-smelling, purulent drainage and surrounding erythema. Culture of the abscess fluid grows several bacteria species, including gram-negative, anaerobic, non-spore-forming bacilli that are resistant to bile and aminoglycoside antibiotics. Which of the following is the most likely source of this genus of bacteria?
Q19
A previously healthy 17-year-old boy is brought to the emergency department because of fever, nausea, and myalgia for the past day. His temperature is 39.5°C (103.1°F), pulse is 112/min, and blood pressure is 77/55 mm Hg. Physical examination shows scattered petechiae over the anterior chest and abdomen. Blood culture grows an organism on Thayer-Martin agar. Which of the following virulence factors of the causal organism is most likely responsible for the high mortality rate associated with it?
Q20
A 20-year-old man comes to the physician because of a 3-day history of fever, myalgia, and swelling in his left groin after a recent camping trip in northern California. He appears acutely ill. Physical examination shows tender, left-sided inguinal lymphadenopathy and an enlarged, tender lymph node in the right axilla that is draining bloody necrotic material. Microscopic examination of a lymph node aspirate shows gram-negative coccobacilli with bipolar staining and a safety-pin appearance. This patient's condition is most likely caused by an organism with which of the following reservoirs?
Gram-negative US Medical PG Practice Questions and MCQs
Question 11: A 10-year-old girl is brought to the emergency department because of a 2-day history of bloody diarrhea and abdominal pain. Four days ago, she visited a petting zoo with her family. Her temperature is 39.4°C (102.9°F). Abdominal examination shows tenderness to palpation of the right lower quadrant. Stool cultures at 42°C grow colonies that turn black after adding phenylenediamine. Which of the following best describes the most likely causal organism?
A. Gram-positive, anaerobic, rod-shaped bacteria that form spores
B. Gram-positive, aerobic, rod-shaped bacteria that produce catalase
C. Gram-negative, non-flagellated bacteria that do not ferment lactose
D. Gram-negative, flagellated bacteria that do not ferment lactose (Correct Answer)
E. Gram-negative, non-flagellated bacteria that ferment lactose
Explanation: ***Gram-negative, flagellated bacteria that do not ferment lactose***
- The clinical presentation of **bloody diarrhea**, **abdominal pain**, and fever, along with a history of **petting zoo exposure**, strongly suggests a *Campylobacter* infection, which is a **gram-negative, flagellated, curved rod** that does not ferment lactose.
- The growth at **42°C (thermophilic)** and a **positive oxidase test** (indicated by colonies turning black after adding phenylenediamine, an oxidase reagent) are characteristic features of *Campylobacter spp*.
*Gram-positive, anaerobic, rod-shaped bacteria that form spores*
- This description typically refers to organisms like *Clostridium difficile* or *Clostridium perfringens*, which can cause diarrhea.
- However, they are **anaerobic** and would not grow well in typical stool culture conditions without specific anaerobic techniques, nor would they produce a positive oxidase test.
*Gram-positive, aerobic, rod-shaped bacteria that produce catalase*
- This describes organisms like *Listeria monocytogenes* or *Bacillus cereus*.
- While *Listeria* can cause gastrointestinal symptoms, it's less commonly associated with the acute, bloody diarrhea and petting zoo exposure seen here, and *Bacillus cereus* typically causes food poisoning with vomiting.
*Gram-negative, non-flagellated bacteria that do not ferment lactose*
- This description commonly applies to *Shigella spp.*
- While *Shigella* causes **bloody diarrhea** and **abdominal pain**, it is typically **non-motile** (non-flagellated), whereas *Campylobacter* is motile due to its flagella.
*Gram-negative, non-flagellated bacteria that ferment lactose*
- This description would fit organisms like enteropathogenic *E. coli* (EPEC) or enterotoxigenic *E. coli* (ETEC).
- However, the specific growth conditions (thermophilic) and positive oxidase test pointed to by phenylenediamine reactivity are not characteristic of these organisms.
Question 12: A 29-year-old man comes to the physician because of a 3-day history of a swollen right knee. Over the past several weeks, he has had similar episodes affecting the right knee and sometimes also the left elbow, in which the swelling lasted an average of 5 days. He has a history of a rash that subsided 2 months ago. He lives in Connecticut with his wife and works as a landscaper. His temperature is 37.8°C (100°F), pulse is 90/min, respirations are 12/min, and blood pressure is 110/75 mm Hg. Physical examination shows a tender and warm right knee; range of motion is limited by pain. The remainder of the examination shows no abnormalities. His hematocrit is 44%, leukocyte count is 10,300/mm3, and platelet count is 145,000/mm3. Serum electrolyte concentrations are within normal limits. Arthrocentesis is performed and the synovial fluid is cloudy. Gram stain is negative. Analysis of the synovial fluid shows a leukocyte count of 70,000/mm3 and 80% neutrophils. Serologic testing confirms the diagnosis. Which of the following is the most likely cause?
A. Rheumatoid arthritis
B. Neisseria gonorrhoeae
C. Borrelia burgdorferi (Correct Answer)
D. Campylobacter jejuni
E. Osteoarthritis
Explanation: ***Borrelia burgdorferi***
- The patient's **migratory polyarthritis** (affecting knee and elbow intermittently), history of a **rash** (consistent with erythema migrans), and residence in an **endemic area** (Connecticut) strongly suggest **Lyme disease**.
- **Synovial fluid analysis** showing high leukocyte count with neutrophilic predominance is typical of inflammatory arthritis, including Lyme arthritis, and **serologic testing** will confirm the presence of *Borrelia burgdorferi* antibodies.
*Rheumatoid arthritis*
- While rheumatoid arthritis causes inflammatory polyarthritis, it typically presents with **symmetrical joint involvement**, morning stiffness, and often involves smaller joints first, which is not described.
- The presence of a preceding **rash** and resolution within weeks is not characteristic of rheumatoid arthritis.
*Neisseria gonorrhoeae*
- **Disseminated gonococcal infection** can cause migratory polyarthralgia or septic arthritis, but it is typically associated with a history of recent unprotected sexual activity and often with tenosynovitis or dermatitis (pustular or vesicular lesions).
- While gram stain is negative in this case, gonococcal arthritis usually has a more rapid onset and systemic symptoms.
*Campylobacter jejuni*
- *Campylobacter jejuni* is a common cause of **reactive arthritis**, which can cause inflammatory joint pain after a gastrointestinal infection.
- However, reactive arthritis typically involves the **lower extremities** and has a specific pattern of oligoarthritis, often with enthesitis or dactylitis, and the preceding rash and geographical factors do not fit.
*Osteoarthritis*
- Osteoarthritis is a **degenerative joint disease** characterized by pain that worsens with activity and improves with rest, and typically affects older individuals.
- It does not present with a preceding **rash**, migratory inflammatory episodes, or a highly inflammatory synovial fluid (high leukocyte count with neutrophilic predominance).
Question 13: An immunology expert is explaining the functions of macrophages to biology students. He describes a hypothetical case scenario as follows: a potentially harmful gram-negative bacillus encounters a macrophage in the tissues. The Toll-like receptor (TLR) on the macrophage recognizes the bacterial lipopolysaccharide (LPS). The macrophage is activated by the binding of TLR with bacterial LPS and by interferon-γ (IFN-γ). Which of the following cytokines is most likely to be secreted by the activated macrophage?
A. Interleukin-4 (IL-4)
B. Interleukin-1 receptor antagonist (IL-1RA)
C. Interleukin-10 (IL-10)
D. Interleukin-2 (IL-2)
E. Interleukin-12 (IL-12) (Correct Answer)
Explanation: ***Interleukin-12 (IL-12)***
- Macrophage activation by **LPS (a PAMP recognized by TLR4)** and **IFN-γ (a macrophage-activating cytokine)** leads to the secretion of pro-inflammatory cytokines, with **IL-12** being a key mediator.
- **IL-12** is crucial for promoting **Th1 differentiation** and enhancing **NK cell activity**, thus linking innate and adaptive immunity against intracellular pathogens and tumor cells.
*Interleukin-4 (IL-4)*
- **IL-4** is primarily secreted by **Th2 cells, mast cells, and basophils**, and is involved in **allergic responses** and humoral immunity.
- It promotes **Th2 differentiation** and IgE production, which is not the primary response to a Gram-negative bacterial encounter as described.
*Interleukin-1 receptor antagonist (IL-1RA)*
- **IL-1RA** is an **anti-inflammatory cytokine** that blocks the effects of IL-1α and IL-1β, thereby downregulating inflammatory responses.
- While it can be produced by macrophages, initial activation by LPS and IFN-γ would trigger pro-inflammatory mediators before the release of antagonists to temper the response.
*Interleukin-10 (IL-10)*
- **IL-10** is a potent **anti-inflammatory cytokine** that suppresses the immune response, particularly by inhibiting cytokine production by macrophages and Th1 cells.
- Macrophage activation in response to a pathogen initially leads to pro-inflammatory cytokine secretion, with IL-10 typically arising later to resolve the inflammation.
*Interleukin-2 (IL-2)*
- **IL-2** is predominantly produced by **T lymphocytes**, especially **Th1 cells**, upon activation.
- Its main roles include **T cell proliferation**, differentiation, and maintenance of regulatory T cells, rather than being a primary cytokine secreted by activated macrophages in this context.
Question 14: An 8-year-old boy is brought to the emergency department because of a 4-day history of severe, left-sided ear pain and purulent discharge from his left ear. One week ago, he returned with his family from their annual summer vacation at a lakeside cabin, where he spent most of the time outdoors hiking and swimming. Examination shows tragal tenderness and a markedly edematous and erythematous external auditory canal. Audiometry shows conductive hearing loss of the left ear. Which of the following is the most likely cause of this patient's symptoms?
A. Pleomorphic replacement of normal bone
B. Abnormal epithelial growth on tympanic membrane
C. Infection with Pseudomonas aeruginosa (Correct Answer)
D. Infection with varicella zoster virus
E. Infection with Aspergillus species
Explanation: ***Infection with Pseudomonas aeruginosa***
- The patient's history of swimming, followed by severe ear pain, purulent discharge, tragal tenderness, and an edematous external auditory canal, are classic signs of **otitis externa** (swimmer's ear).
- **Pseudomonas aeruginosa** is the most common bacterial cause of otitis externa, thriving in moist environments.
*Pleomorphic replacement of normal bone*
- This describes features more consistent with a **bony tumor** or a severe, chronic infection leading to bone erosion, which is not typically seen in acute otitis externa.
- While otitis externa can become severe, the initial presentation here strongly points to an acute infectious process rather than a neoplastic transformation.
*Abnormal epithelial growth on tympanic membrane*
- This description suggests a **cholesteatoma**, which is an abnormal skin growth behind the eardrum.
- Cholesteatomas typically cause hearing loss and chronic ear discharge but are not usually associated with the acute onset of severe pain and tragal tenderness typical of otitis externa.
*Infection with varicella zoster virus*
- An infection with **varicella zoster virus** in the ear would cause **Ramsay Hunt syndrome**, characterized by severe ear pain, vesicular rash on the ear or in the auditory canal, facial paralysis, and hearing loss.
- The absence of a vesicular rash and facial paralysis makes this diagnosis unlikely.
*Infection with Aspergillus species*
- While fungal infections (like **otomycosis** caused by Aspergillus) can occur in the ear, they are less common than bacterial infections and typically present with symptoms such as itching, aural fullness, and a white or black fungal debris.
- The described purulent discharge and severe pain are more characteristic of a bacterial infection.
Question 15: An 18-year-old college student seeks evaluation at an emergency department with complaints of fevers with chills, fatigue, diarrhea, and loss of appetite, which have lasted for 1 week. He says that his symptoms are progressively getting worse. He was taking over-the-counter acetaminophen, but it was ineffective. The past medical history is insignificant. His temperature is 38.8°C (101.9°F) and his blood pressure is 100/65 mm Hg. The physical examination is within normal limits, except that the patient appears ill. Eventually, a diagnosis of typhoid fever was established and he is started on appropriate antibiotics. Which of the following cellular components is most likely to be responsible for the toxic symptoms in this patient?
A. Pili on the bacterial cell surface
B. Lipid A - a toxic component present in the bacterial cell wall (Correct Answer)
C. Flagella
D. Toxins secreted by the bacteria
E. Outer capsule
Explanation: ***Lipid A - a toxic component present in the bacterial cell wall***
- The toxic symptoms of typhoid fever, caused by *Salmonella Typhi*, are primarily due to **endotoxins**. **Lipid A** is the toxic component of **lipopolysaccharide (LPS)**, which is an endotoxin found in the outer membrane of Gram-negative bacteria like *Salmonella*.
- When bacteria are lysed, LPS is released, triggering a strong immune response that leads to fever, chills, hypotension, and systemic inflammation characteristic of **septic shock** or severe infections.
*Pili on the bacterial cell surface*
- **Pili** (fimbriae) are hair-like appendages responsible for **bacterial adherence** to host cells and surfaces, facilitating colonization.
- While important for establishing infection, pili themselves are not directly responsible for the **toxic systemic symptoms** like fever and hypotension.
*Flagella*
- **Flagella** are whip-like structures primarily involved in **bacterial motility**, allowing the bacteria to move through fluids.
- They are essential for bacterial dissemination within the host but do not directly cause the **toxic effects** associated with severe systemic infections.
*Toxins secreted by the bacteria*
- While some bacteria secrete exotoxins that cause disease, the primary toxic component in **Gram-negative infections** like typhoid fever is the **endotoxin (LPS)**, which is part of the cell wall and released upon bacterial lysis, not actively secreted from living bacteria.
- *Salmonella Typhi* does not produce potent exotoxins that cause the main systemic manifestations observed in typhoid fever.
*Outer capsule*
- The **outer capsule** is a protective layer that helps bacteria evade phagocytosis and contributes to **virulence** by offering immune evasion.
- While critical for bacterial survival and pathogenicity, the capsule itself is not directly responsible for triggering the **toxic inflammatory response** that causes the symptoms of septic shock.
Question 16: A stool sample was taken from a 19-year-old male who presented with profuse watery diarrhea. He recently returned from a trip to Central America. A microbiologist identified the causative agent as a gram-negative, oxidase-positive, comma-shaped bacteria that is able to grow well in a pH > 8. Which of the following is a mechanism of action of the toxin produced by this bacteria?
A. Overactivation of adenylate cyclase by inhibition of Gi subunit by ADP-ribosylation
B. Inactivation of the 60S ribosomal subunit by cleaving an adenine from the 28S rRNA
C. Overactivation of guanylate cyclase
D. Overactivation of adenylate cyclase by activation of Gs subunit by ADP-ribosylation (Correct Answer)
E. Degradation of cell membranes by hydrolysis of the phospholipids
Explanation: ***Overactivation of adenylate cyclase by activation of Gs subunit by ADP-ribosylation***
- The description of the bacterium as **gram-negative, oxidase-positive, comma-shaped, growing well in pH > 8**, and causing **profuse watery diarrhea** after travel to Central America points to *Vibrio cholerae*.
- **Cholera toxin** (CTX) produced by *V. cholerae* is an A-B toxin that **ADP-ribosylates the Gs α-subunit**, permanently activating **adenylate cyclase**. This leads to increased cAMP levels, causing secretion of water and electrolytes into the intestinal lumen.
*Overactivation of adenylate cyclase by inhibition of Gi subunit by ADP-ribosylation*
- This mechanism describes the action of **pertussis toxin** from *Bordetella pertussis*, which ADP-ribosylates and **inhibits the Gi subunit**, preventing adenylate cyclase inhibition.
- While both ultimately increase cAMP, the specific target and mechanism (inhibition of Gi vs. activation of Gs) differ from cholera toxin.
*Inactivation of the 60S ribosomal subunit by cleaving an adenine from the 28S rRNA*
- This mechanism is characteristic of **Shiga toxin** produced by *Shigella dysenteriae* and Shiga-like toxins (verotoxins) produced by **enterohemorrhagic *E. coli*** (EHEC).
- These toxins inhibit protein synthesis, leading to cell death and often bloody diarrhea and hemolytic uremic syndrome, which is not described here.
*Overactivation of guanylate cyclase*
- **Heat-stable enterotoxins (ST)** produced by **enterotoxigenic *E. coli*** (ETEC) activate **guanylate cyclase**, leading to increased cGMP and subsequent fluid secretion.
- While ETEC can cause watery diarrhea, the bacterial characteristics provided (oxidase-positive, comma-shaped) do not fit *E. coli*.
*Degradation of cell membranes by hydrolysis of the phospholipids*
- This mechanism is associated with toxins like **phospholipases** or **lecithinases** (e.g., alpha-toxin of *Clostridium perfringens*).
- These toxins cause direct cell lysis and tissue damage, which is not the primary mechanism of action for the watery diarrhea seen in cholera.
Question 17: A 14-year-old boy presents with abdominal pain and diarrhea after returning from an East Asian vacation. Stool sample reveals the presence of red and white blood cells. Stool culture shows growth of immobile, non-lactose fermenting gram-negative rods. The attending physician explains to the medical students that the bacteria function by invading intestinal M-cells. The bacterium responsible for this patient's infection is:
A. Shigella dysenteriae (Correct Answer)
B. Salmonella enteritidis
C. Helicobacter pylori
D. Escherichia coli
E. Vibrio cholerae
Explanation: ***Shigella dysenteriae***
- The combination of **abdominal pain**, **bloody diarrhea** (red and white blood cells in stool), **immobile, non-lactose fermenting gram-negative rods**, and **invasion of M-cells** is classic for *Shigella* infection.
- *Shigella* species, particularly *S. dysenteriae*, cause **dysentery** by directly invading and destroying the intestinal epithelium, often in M-cells, leading to inflammation and ulceration.
*Salmonella enteritidis*
- While *Salmonella enteritidis* is also a **non-lactose fermenting gram-negative rod** and can cause diarrhea, it is typically **motile** (unlike the immobile bacteria described) and invades enterocytes, not specifically M-cells for its primary pathogenic mechanism.
- While it can cause bloody diarrhea, the **immotility** and primary M-cell invasion point away from *Salmonella*.
*Helicobacter pylori*
- *Helicobacter pylori* is a **spiral-shaped, gram-negative bacterium** primarily associated with gastritis and peptic ulcers, not acute bloody diarrhea.
- It colonizes the stomach lining and is not characteristically an immobile, non-lactose fermenting rod found in diarrheal stool.
*Vibrio cholerae*
- *Vibrio cholerae* causes **profuse watery diarrhea** (cholera) and is characterized by a **comma-shaped gram-negative rod** that is highly motile.
- It does not cause bloody diarrhea or invade M-cells; its pathogenicity is due to the production of an enterotoxin.
*Escherichia coli*
- While *E. coli* is a **gram-negative rod** and some strains can cause diarrhea (e.g., EHEC, ETEC), most strains are **lactose fermenting**.
- Pathogenic *E. coli* strains have various mechanisms, but the specific combination of **immobile, non-lactose fermenting rods with M-cell invasion** leading to dysentery is not characteristic of common diarrheagenic *E. coli*.
Question 18: A 62-year-old woman with type 2 diabetes mellitus is brought to the emergency department by her husband because of fever, chills, and purulent drainage from a foot ulcer for 2 days. Her hemoglobin A1c was 15.4% 16 weeks ago. Physical examination shows a 2-cm ulcer on the plantar surface of the left foot with foul-smelling, purulent drainage and surrounding erythema. Culture of the abscess fluid grows several bacteria species, including gram-negative, anaerobic, non-spore-forming bacilli that are resistant to bile and aminoglycoside antibiotics. Which of the following is the most likely source of this genus of bacteria?
A. Stomach
B. Oropharynx
C. Vagina
D. Colon (Correct Answer)
E. Skin
Explanation: ***Colon***
- The description of the bacteria—**gram-negative, anaerobic, non-spore-forming bacilli** that are **resistant to bile** and **aminoglycoside antibiotics**—is highly characteristic of the genus *Bacteroides*, especially *Bacteroides fragilis*.
- *Bacteroides fragilis* is a prominent component of the normal **colonic microflora** and is frequently implicated in infections originating from breaches in the gastrointestinal tract, such as a diabetic foot ulcer with a mixed infection.
*Stomach*
- The stomach's highly acidic environment generally limits significant bacterial colonization, and it is not a primary source of mixed anaerobic infections as described.
- While *Helicobacter pylori* can colonize the stomach, it does not fit the described microbiological characteristics.
*Oropharynx*
- The oropharynx contains a diverse microbiota, including anaerobes like **Peptostreptococcus** and **Fusobacterium**, but it is not the typical source for *Bacteroides fragilis* or the specific resistance profile mentioned.
- Oropharyngeal anaerobes are more commonly associated with head and neck infections, aspiration pneumonia, or dental abscesses.
*Vagina*
- The vaginal flora includes various anaerobes such as **Gardnerella vaginalis** and some *Bacteroides* species, but it is not the most common or primary source of widespread mixed anaerobic infections matching this description.
- Infections originating from the vagina would typically be linked to pelvic or genitourinary conditions.
*Skin*
- The skin surface predominantly harbors **aerobic** and **facultative anaerobic bacteria** like **Staphylococcus** and **Streptococcus** species.
- While skin breaches can lead to infections, the described **anaerobic, gram-negative, bile-resistant** profile points away from the typical skin flora as the primary source for the specific bacterial characteristics given.
Question 19: A previously healthy 17-year-old boy is brought to the emergency department because of fever, nausea, and myalgia for the past day. His temperature is 39.5°C (103.1°F), pulse is 112/min, and blood pressure is 77/55 mm Hg. Physical examination shows scattered petechiae over the anterior chest and abdomen. Blood culture grows an organism on Thayer-Martin agar. Which of the following virulence factors of the causal organism is most likely responsible for the high mortality rate associated with it?
A. Immunoglobulin A protease
B. Lipooligosaccharide (Correct Answer)
C. Toxic shock syndrome toxin-1
D. Lipoteichoic acid
E. Erythrogenic exotoxin A
Explanation: ***Lipooligosaccharide***
- The patient's presentation with **fever**, **hypotension**, and **petechiae**, along with a positive blood culture on Thayer-Martin agar, points to **meningococcemia** caused by *Neisseria meningitidis*.
- **Lipooligosaccharide (LOS)** acts as an **endotoxin**, triggering an excessive inflammatory response that leads to widespread vascular damage, **capillary leakage**, and **septic shock**, accounting for the high mortality.
*Immunoglobulin A protease*
- While *N. meningitidis* produces **IgA protease** to cleave secretory IgA and evade host defenses on mucosal surfaces, this factor is primarily involved in colonization and initial invasion rather than the systemic severity and mortality of septic shock.
- Its role is to help the bacteria **adhere and penetrate** host mucous membranes, but it does not directly cause the shock and petechiae seen in this severe presentation.
*Toxic shock syndrome toxin-1*
- **Toxic shock syndrome toxin-1 (TSST-1)** is a **superantigen** produced by *Staphylococcus aureus* that causes **toxic shock syndrome**, which can present with fever, rash, and hypotension.
- However, the organism grown on **Thayer-Martin agar** is characteristic of *Neisseria meningitidis*, not *Staphylococcus aureus*.
*Lipoteichoic acid*
- **Lipoteichoic acid** is a major component of the cell wall of **Gram-positive bacteria**, acting as a potent proinflammatory molecule and contributing to septic shock in those infections.
- *Neisseria meningitidis* is a **Gram-negative bacterium**, and therefore does not possess lipoteichoic acid.
*Erythrogenic exotoxin A*
- **Erythrogenic exotoxin A** is primarily produced by ***Streptococcus pyogenes*** and is responsible for the characteristic rash of **scarlet fever**.
- While *S. pyogenes* can cause invasive infections, the clinical picture and the specific growth on **Thayer-Martin agar** are not consistent with streptococcal infection.
Question 20: A 20-year-old man comes to the physician because of a 3-day history of fever, myalgia, and swelling in his left groin after a recent camping trip in northern California. He appears acutely ill. Physical examination shows tender, left-sided inguinal lymphadenopathy and an enlarged, tender lymph node in the right axilla that is draining bloody necrotic material. Microscopic examination of a lymph node aspirate shows gram-negative coccobacilli with bipolar staining and a safety-pin appearance. This patient's condition is most likely caused by an organism with which of the following reservoirs?
A. Squirrels (Correct Answer)
B. Deer
C. Bats
D. Dogs
E. Birds
Explanation: ***Squirrels***
- The clinical presentation of **fever**, **myalgia**, **tender lymphadenopathy (buboes)**, especially with **bloody necrotic material drainage**, in a patient with recent outdoor exposure in **northern California**, is highly suggestive of **bubonic plague**.
- Microscopic examination revealing **gram-negative coccobacilli with bipolar staining** and a **safety-pin appearance** is **pathognomonic for *Yersinia pestis***, the causative agent of plague.
- The primary reservoir for *Y. pestis* is **wild rodents**, particularly **ground squirrels, prairie dogs, and rock squirrels** in the western United States, including California.
- Transmission occurs via flea bites from infected rodents, or through direct contact with infected animals.
*Deer*
- **Deer** are not reservoirs for *Yersinia pestis*.
- Deer serve as reservoirs for **Lyme disease** (*Borrelia burgdorferi*) transmitted by *Ixodes* ticks, which presents with erythema migrans, not buboes with bipolar-staining bacteria.
- Deer may also harbor ticks that transmit other diseases (ehrlichiosis, anaplasmosis), but none match this clinical picture.
*Bats*
- **Bats** are not associated with *Yersinia pestis* infection.
- Bats are reservoirs for **rabies virus** and **Histoplasma capsulatum** (histoplasmosis from bat guano in caves).
- Neither presents with the characteristic bubonic lymphadenopathy and gram-negative coccobacilli with bipolar staining seen here.
*Dogs*
- **Dogs** are not primary reservoirs for plague, though they can become infected and rarely transmit to humans.
- Dogs are reservoirs for **rabies**, **leptospirosis**, and **Capnocytophaga** infections.
- These do not match the clinical presentation of buboes and the pathognomonic microscopic findings of *Y. pestis*.
*Birds*
- **Birds** are not reservoirs for *Yersinia pestis*.
- Birds can harbor **Chlamydophila psittaci** (causing psittacosis/atypical pneumonia) and **Cryptococcus neoformans** (in pigeon droppings).
- These present with respiratory symptoms, not bubonic lymphadenopathy with bipolar-staining bacteria.
