A 45-year-old man comes to the physician because of a 1-day history of progressive pain and blurry vision of his right eye. He has difficulties opening the eye because of pain. His left eye is asymptomatic. He wears contact lenses. He has bronchial asthma treated with inhaled salbutamol. He works as a kindergarten teacher. His temperature is 37°C (98.6°F), pulse is 85/min, and blood pressure is 135/75 mm Hg. Examination shows a visual acuity in the left eye of 20/25 and the ability to count fingers at 3 feet in the right eye. A photograph of the right eye is shown. Which of the following is the most likely diagnosis?
Q2
A 23-year-old man comes to the physician because of a 2-day history of profuse watery diarrhea and abdominal cramps. Four days ago, he returned from a backpacking trip across Southeast Asia. Physical examination shows dry mucous membranes and decreased skin turgor. Stool culture shows gram-negative, oxidase-positive, curved rods that have a single polar flagellum. The pathogen responsible for this patient's condition most likely has which of the following characteristics?
Q3
An 83-year-old male presents to the emergency department with altered mental status. The patient’s vitals signs are as follows: temperature is 100.7 deg F (38.2 deg C), blood pressure is 143/68 mmHg, heart rate is 102/min, and respirations are 22/min. The caretaker states that the patient is usually incontinent of urine, but she has not seen any soiled adult diapers in the past 48 hours. A foley catheter is placed with immediate return of a large volume of cloudy, pink urine. Which of the following correctly explains the expected findings from this patient’s dipstick urinalysis?
Q4
A 10-year-old boy is brought in to the emergency room by his parents after he complained of being very weak during a soccer match the same day. The parents noticed that yesterday, the patient seemed somewhat clumsy during soccer practice and was tripping over himself. Today, the patient fell early in his game and complained that he could not get back up. The patient is up-to-date on his vaccinations and has no previous history of illness. The parents do report that the patient had abdominal pain and bloody diarrhea the previous week, but the illness resolved without antibiotics or medical attention. The patient’s temperature is 100.9°F (38.3°C), blood pressure is 110/68 mmHg, pulse is 84/min, and respirations are 14/min. On exam, the patient complains of tingling sensations that seem reduced in his feet. He has no changes in vibration or proprioception. Achilles and patellar reflexes are 1+ bilaterally. On strength testing, foot dorsiflexion and plantar flexion are 3/5 and knee extension and knee flexion are 4-/5. Hip flexion, hip extension, and upper extremity strength are intact. Based on this clinical history and physical exam, what pathogenic agent could have been responsible for the patient’s illness?
Q5
A 4-year-old boy is brought to the physician because of a 1-day history of passing small quantities of dark urine. Two weeks ago, he had fever, abdominal pain, and bloody diarrhea for several days that were treated with oral antibiotics. Physical examination shows pale conjunctivae and scleral icterus. His hemoglobin concentration is 7.5 g/dL, platelet count is 95,000/mm3, and serum creatinine concentration is 1.9 mg/dL. A peripheral blood smear shows irregular red blood cell fragments. Avoiding consumption of which of the following foods would have most likely prevented this patient's condition?
Q6
A 43-year-old man hospitalized for acute pancreatitis develops a high-grade fever and productive cough with gelatinous sputum. A sample of his expectorated sputum is obtained and fixed to a microscope slide using heat. A crystal violet dye is applied to the slide, followed by an iodine solution, acetone solution, and lastly, safranin dye. A photomicrograph of the result is shown. Which of the following cell components is responsible for the pink color seen on this stain?
Q7
An 86-year-old male with a history of hypertension and hyperlipidemia is sent to the hospital from the skilled nursing facility due to fever, confusion, and decreased urine output. Urinalysis shows 12-18 WBC/hpf with occasional lymphocytes. Urine and blood cultures grow out gram-negative, motile, urease positive rods. What component in the identified bacteria is primarily responsible for causing the innate immune response seen in this patient?
Q8
A 46-year-old woman from Ecuador is admitted to the hospital because of tarry-black stools and epigastric pain for 2 weeks. The epigastric pain is relieved after meals, but worsens after 1–2 hours. She has no history of serious illness and takes no medications. Physical examination shows no abnormalities. Fecal occult blood test is positive. Esophagogastroduodenoscopy shows a bleeding duodenal ulcer. Microscopic examination of a duodenal biopsy specimen is most likely to show which of the following?
Q9
A 4-year-old boy is brought to the physician because of a 3-day history of fever and left ear pain. Examination of the left ear shows a bulging tympanic membrane with green discharge. Gram stain of the discharge shows a gram-negative coccobacillus. The isolated organism grows on chocolate agar. The causal pathogen most likely produces a virulence factor that acts by which of the following mechanisms?
Q10
A 24-year-old man comes to the physician with a 2-day history of fever, crampy abdominal pain, and blood-tinged diarrhea. He recently returned from a trip to Mexico. His temperature is 38.2°C (100.8°F). Abdominal examination shows diffuse tenderness to palpation; bowel sounds are hyperactive. Stool cultures grow nonlactose fermenting, oxidase-negative, gram-negative rods that do not produce hydrogen sulfide on triple sugar iron agar. Which of the following processes is most likely involved in the pathogenesis of this patient's condition?
Gram-negative US Medical PG Practice Questions and MCQs
Question 1: A 45-year-old man comes to the physician because of a 1-day history of progressive pain and blurry vision of his right eye. He has difficulties opening the eye because of pain. His left eye is asymptomatic. He wears contact lenses. He has bronchial asthma treated with inhaled salbutamol. He works as a kindergarten teacher. His temperature is 37°C (98.6°F), pulse is 85/min, and blood pressure is 135/75 mm Hg. Examination shows a visual acuity in the left eye of 20/25 and the ability to count fingers at 3 feet in the right eye. A photograph of the right eye is shown. Which of the following is the most likely diagnosis?
A. Angle-closure glaucoma
B. Pseudomonas keratitis (Correct Answer)
C. Herpes simplex keratitis
D. Staphylococcus aureus keratitis
E. Herpes zoster keratitis
Explanation: **Pseudomonas keratitis**
- This patient's history of **contact lens** use, acute onset of **severe pain**, **blurry vision**, and the provided image (which likely shows a **corneal ulcer** with associated hypopyon or dense infiltrate) are highly suggestive of a bacterial keratitis, particularly *Pseudomonas*.
- *Pseudomonas aeruginosa* is a common and aggressive cause of **contact lens-associated keratitis**, leading to rapid progression, significant vision loss, and often severe pain.
*Angle-closure glaucoma*
- While it presents with **acute eye pain** and **blurry vision**, it typically involves a **red eye** with a **dilated pupil** and a **hard globe on palpation**, and generally no visible corneal infiltrate as implied by the image.
- The primary mechanism is increased **intraocular pressure** due to blocked fluid outflow, not an infectious process of the cornea.
*Herpes simplex keratitis*
- Often presents with a characteristic **dendritic corneal ulcer** pattern, which is usually less acutely painful and rapid in progression compared to severe bacterial infections unless superinfected.
- Recurrent episodes are common, and the vision loss might not be as profound initially as observed here.
*Staphylococcus aureus keratitis*
- While *S. aureus* can cause **bacterial keratitis**, especially in contact lens wearers, the severe, rapidly progressive nature and profound vision loss described are more classically associated with the highly virulent *Pseudomonas aeruginosa* in this context.
- Differentiation often requires culture, but the clinical picture leans towards *Pseudomonas* given the aggressive presentation.
*Herpes zoster keratitis*
- This condition is caused by the reactivation of the **varicella-zoster virus** and typically presents with a **rash in a dermatomal distribution** (V1 ophthalmic division) affecting the forehead and eyelids, along with eye symptoms.
- The absence of a characteristic vesicular rash and the primary focus on the globe make this less likely.
Question 2: A 23-year-old man comes to the physician because of a 2-day history of profuse watery diarrhea and abdominal cramps. Four days ago, he returned from a backpacking trip across Southeast Asia. Physical examination shows dry mucous membranes and decreased skin turgor. Stool culture shows gram-negative, oxidase-positive, curved rods that have a single polar flagellum. The pathogen responsible for this patient's condition most likely has which of the following characteristics?
A. Acts by activation of guanylate cyclase
B. Forms spores in unfavorable environment
C. Grows well in medium with pH of 9 (Correct Answer)
D. Infection commonly precedes Guillain-Barré syndrome
E. Causes necrosis of Peyer patches of distal ileum
Explanation: ***Grows well in medium with pH of 9***
- The clinical presentation with **profuse watery diarrhea** after travel to Southeast Asia, along with the finding of **gram-negative, oxidase-positive, curved rods** with a **single polar flagellum**, is highly suggestive of **_Vibrio cholerae_**.
- _Vibrio cholerae_ is known for its ability to **grow well in alkaline environments**, such as a medium with a pH of 9, which distinguishes it from many other enteric pathogens.
*Acts by activation of guanylate cyclase*
- This mechanism of action is characteristic of **heat-stable enterotoxins (ST)** produced by **enterotoxigenic _Escherichia coli_ (ETEC)**, which cause traveler's diarrhea.
- While ETEC can cause watery diarrhea, _Vibrio cholerae_ primarily acts by activating **adenylate cyclase** through its cholera toxin, not guanylate cyclase.
*Forms spores in unfavorable environment*
- The ability to form **spores** is a characteristic feature of certain **Gram-positive bacteria**, notably _Bacillus_ and _Clostridium_ species.
- **Gram-negative rods** like _Vibrio cholerae_ do not form spores as a survival mechanism in unfavorable conditions.
*Infection commonly precedes Guillain-Barré syndrome*
- **_Campylobacter jejuni_** infection is a well-known precursor to **Guillain-Barré syndrome (GBS)** due to molecular mimicry between _Campylobacter_ lipo-oligosaccharides and gangliosides in peripheral nerves.
- While _Campylobacter_ can cause watery diarrhea and is a curved rod, the description of **profuse watery diarrhea** and good growth in alkaline conditions points more strongly to _Vibrio cholerae_ rather than _Campylobacter_.
*Causes necrosis of Peyer patches of distal ileum*
- **Necrosis of Peyer patches** in the distal ileum is a characteristic pathological feature of **typhoid fever**, caused by **_Salmonella Typhi_**.
- The presentation of **profuse watery diarrhea** and the microbiological description do not align with typhoid fever, which typically presents with fever, malaise, and constipation or pea-soup diarrhea.
Question 3: An 83-year-old male presents to the emergency department with altered mental status. The patient’s vitals signs are as follows: temperature is 100.7 deg F (38.2 deg C), blood pressure is 143/68 mmHg, heart rate is 102/min, and respirations are 22/min. The caretaker states that the patient is usually incontinent of urine, but she has not seen any soiled adult diapers in the past 48 hours. A foley catheter is placed with immediate return of a large volume of cloudy, pink urine. Which of the following correctly explains the expected findings from this patient’s dipstick urinalysis?
A. Detection of an enzyme produced by white blood cells
B. Detection of urinary nitrate conversion by gram-negative pathogens (Correct Answer)
C. Detection of an enzyme produced by red blood cells
D. Detection of urinary nitrate conversion by gram-positive pathogens
E. Direct detection of white blood cell surface proteins
Explanation: ***Detection of urinary nitrate conversion by gram-negative pathogens***
- The presence of **nitrites** on a urine dipstick is a highly specific indicator of a **urinary tract infection (UTI)** caused by **gram-negative bacteria**.
- **Gram-negative bacteria** like *E. coli* possess an enzyme, **nitrate reductase**, which converts urinary nitrates (normally present from dietary intake) into nitrites.
- This is the **most specific finding** for gram-negative UTI and directly explains the expected dipstick result in this patient with cloudy urine and clinical signs of infection.
*Detection of an enzyme produced by white blood cells*
- This refers to the detection of **leukocyte esterase**, an enzyme released by neutrophils (white blood cells) in response to infection or inflammation.
- While **leukocyte esterase** would likely be positive in this case of UTI, it is **less specific** than nitrite detection because it can be positive in any inflammatory condition of the urinary tract, not just bacterial infections.
- The **nitrite test** is more specific for identifying **gram-negative bacterial** infections, which are the most common cause of UTIs.
*Detection of an enzyme produced by red blood cells*
- This refers to the detection of **hemoglobin**, which can be indirectly detected by dipstick due to its peroxidase-like activity. While the patient has **pink urine** (indicating hematuria), this finding is less specific for a **bacterial UTI** than nitrites and does not explain the *cause* of the infection.
- Hematuria can be caused by various factors, including irritation from infection, kidney stones, trauma, or malignancy, and doesn't directly point to the type of pathogen.
*Detection of urinary nitrate conversion by gram-positive pathogens*
- **Gram-positive pathogens**, such as *Staphylococcus saprophyticus* or *Enterococcus faecalis*, which can cause UTIs, typically **do not convert urinary nitrates to nitrites** because they lack nitrate reductase enzyme.
- Therefore, a positive nitrite test generally rules out a gram-positive infection as the sole cause of the positive dipstick finding.
*Direct detection of white blood cell surface proteins*
- The dipstick test for **leukocytes** (white blood cells) detects **leukocyte esterase**, an enzyme *released by* neutrophils, not their surface proteins directly.
- While **leukocyte esterase** would likely be positive in this case, a positive **nitrite** test is more specific to the type of bacterial infection (gram-negative) responsible for the majority of UTIs.
Question 4: A 10-year-old boy is brought in to the emergency room by his parents after he complained of being very weak during a soccer match the same day. The parents noticed that yesterday, the patient seemed somewhat clumsy during soccer practice and was tripping over himself. Today, the patient fell early in his game and complained that he could not get back up. The patient is up-to-date on his vaccinations and has no previous history of illness. The parents do report that the patient had abdominal pain and bloody diarrhea the previous week, but the illness resolved without antibiotics or medical attention. The patient’s temperature is 100.9°F (38.3°C), blood pressure is 110/68 mmHg, pulse is 84/min, and respirations are 14/min. On exam, the patient complains of tingling sensations that seem reduced in his feet. He has no changes in vibration or proprioception. Achilles and patellar reflexes are 1+ bilaterally. On strength testing, foot dorsiflexion and plantar flexion are 3/5 and knee extension and knee flexion are 4-/5. Hip flexion, hip extension, and upper extremity strength are intact. Based on this clinical history and physical exam, what pathogenic agent could have been responsible for the patient’s illness?
A. Gram-negative, oxidase-negative, bacillus without hydrogen sulfide gas production
B. Gram-negative, oxidase-positive bacillus
C. Gram-negative, oxidase-positive, comma-shaped bacteria (Correct Answer)
D. Gram-negative, oxidase-negative, bacillus with hydrogen sulfide gas production
E. Gram-positive bacillus
Explanation: ***Gram-negative, oxidase-positive, comma-shaped bacteria***
- The patient's presentation of **ascending weakness**, **tingling sensations (paresthesias)**, and **diminished reflexes** following a diarrheal illness is highly suggestive of **Guillain-Barré Syndrome (GBS)**.
- **_Campylobacter jejuni_**, a **Gram-negative, oxidase-positive, comma-shaped bacteria**, is the most common antecedent infection leading to GBS through molecular mimicry with myelin gangliosides.
*Gram-negative, oxidase-negative, bacillus without hydrogen sulfide gas production*
- This description commonly refers to organisms like **_Shigella_** or **_Escherichia coli_ (EHEC)**.
- While these can cause bloody diarrhea, they are less frequently associated with post-infectious GBS compared to _Campylobacter jejuni_.
*Gram-negative, oxidase-positive bacillus*
- This general description could fit bacteria such as _Pseudomonas aeruginosa_ or _Vibrio cholerae_ (although _Vibrio_ is more specifically comma-shaped).
- While _Vibrio_ can cause diarrheal illness, _Campylobacter_ is a more classic and frequent trigger for GBS.
*Gram-negative, oxidase-negative, bacillus with hydrogen sulfide gas production*
- This characterizes bacteria like **_Salmonella_ species**.
- While **_Salmonella_ enteritis can cause diarrheal illness, it is a less common antecedent infection for GBS compared to _Campylobacter jejuni_**.
*Gram-positive bacillus*
- **Gram-positive bacilli** include organisms like _Clostridium difficile_ (which causes pseudomembranous colitis) or _Listeria monocytogenes_.
- These are not typically associated with bloody diarrhea followed by acute ascending paralysis and GBS.
Question 5: A 4-year-old boy is brought to the physician because of a 1-day history of passing small quantities of dark urine. Two weeks ago, he had fever, abdominal pain, and bloody diarrhea for several days that were treated with oral antibiotics. Physical examination shows pale conjunctivae and scleral icterus. His hemoglobin concentration is 7.5 g/dL, platelet count is 95,000/mm3, and serum creatinine concentration is 1.9 mg/dL. A peripheral blood smear shows irregular red blood cell fragments. Avoiding consumption of which of the following foods would have most likely prevented this patient's condition?
A. Undercooked beef (Correct Answer)
B. Mushrooms
C. Shellfish
D. Canned carrots
E. Raw pork
Explanation: ***Undercooked beef***
- The patient's symptoms (hemolytic anemia, thrombocytopenia, acute kidney injury) following a diarrheal illness are characteristic of **hemolytic uremic syndrome (HUS)**, most commonly caused by **Shiga toxin-producing E. coli (STEC)**, particularly serotype O157:H7.
- STEC infections are frequently acquired through consumption of **undercooked ground beef** or contaminated produce.
- **Note**: Antibiotic treatment of STEC gastroenteritis (as in this case) is associated with **increased risk of HUS** due to enhanced Shiga toxin release, making prevention through proper food handling even more critical.
*Mushrooms*
- While some mushrooms can be poisonous, they typically cause **gastrointestinal upset**, hepatotoxicity, or neurotoxicity, not the specific triad of HUS (hemolytic anemia, thrombocytopenia, and renal failure).
- Mushroom poisoning does not typically lead to the characteristic **microangiopathic hemolytic anemia** with schistocytes.
*Shellfish*
- Contaminated shellfish can cause various illnesses, including **bacterial infections** (e.g., Vibrio) or **paralytic shellfish poisoning** (neurotoxins).
- These conditions do not typically lead to the development of HUS with its specific hematologic and renal manifestations.
*Canned carrots*
- **Botulism** is associated with improperly canned foods, but it presents with **flaccid paralysis** and other neurologic symptoms, not HUS.
- Canned carrots are not a common source of pathogens leading to HUS.
*Raw pork*
- Raw pork is associated with infections like **trichinellosis** or **Taenia solium** (cysticercosis).
- These infections do not cause the constellation of symptoms defining HUS, particularly the **microangiopathic hemolytic anemia** and acute kidney injury.
Question 6: A 43-year-old man hospitalized for acute pancreatitis develops a high-grade fever and productive cough with gelatinous sputum. A sample of his expectorated sputum is obtained and fixed to a microscope slide using heat. A crystal violet dye is applied to the slide, followed by an iodine solution, acetone solution, and lastly, safranin dye. A photomicrograph of the result is shown. Which of the following cell components is responsible for the pink color seen on this stain?
A. Protein
B. Peptidoglycan (Correct Answer)
C. Mycolic acid
D. Glycogen
E. Capsular polysaccharide
Explanation: ***Peptidoglycan***
- The pink color in Gram-negative bacteria results from the **thin peptidoglycan layer** in their cell wall, which is the key structural difference from Gram-positive bacteria.
- During the Gram stain procedure, the **thin peptidoglycan layer** (only 1-2 layers) does not effectively retain the crystal violet-iodine complex after decolorization with acetone or alcohol.
- Once the crystal violet is washed out, the **safranin counterstain binds to the bacterial cell**, producing the characteristic pink/red color. The thin peptidoglycan is thus responsible for the Gram-negative phenotype and pink appearance.
- In this case, the gelatinous sputum suggests *Klebsiella pneumoniae*, a classic Gram-negative rod causing pneumonia in hospitalized patients.
*Protein*
- While proteins are abundant in bacterial cells (ribosomes, enzymes, structural proteins), they do not determine the Gram stain reaction.
- The Gram stain specifically differentiates bacteria based on **cell wall structure**, not general protein content.
*Mycolic acid*
- **Mycolic acid** is a unique long-chain fatty acid found in the cell walls of *Mycobacterium* and *Nocardia* species.
- These acid-fast bacteria require special staining (Ziehl-Neelsen or Kinyoun stain) and do not Gram stain reliably due to their waxy cell wall preventing dye penetration.
*Glycogen*
- **Glycogen** is a polysaccharide storage molecule found in some bacteria and eukaryotic cells.
- It is not a structural component of the bacterial cell wall and plays no role in Gram staining characteristics.
*Capsular polysaccharide*
- The **polysaccharide capsule** is an external layer that enhances virulence by preventing phagocytosis (*Klebsiella* has a prominent capsule causing the gelatinous appearance).
- While present in this organism, the capsule lies **outside the cell wall** and does not participate in the Gram stain mechanism. Special stains (India ink, quellung reaction) are used to visualize capsules.
Question 7: An 86-year-old male with a history of hypertension and hyperlipidemia is sent to the hospital from the skilled nursing facility due to fever, confusion, and decreased urine output. Urinalysis shows 12-18 WBC/hpf with occasional lymphocytes. Urine and blood cultures grow out gram-negative, motile, urease positive rods. What component in the identified bacteria is primarily responsible for causing the innate immune response seen in this patient?
A. Outer membrane (Correct Answer)
B. Nucleic acid
C. Secreted toxin
D. Polyribosylribitol phosphate
E. Teichoic acid in the cell wall
Explanation: ***Correct: Outer membrane***
- The described bacterium (gram-negative, motile, urease-positive rods) is most likely *Proteus mirabilis*, a common cause of UTIs, especially in catheterized or institutionalized patients.
- Gram-negative bacteria possess an **outer membrane** containing **lipopolysaccharide (LPS)**. The lipid A component of LPS is a potent **endotoxin** that triggers a strong **innate immune response** through **Toll-like receptor 4 (TLR4)**, leading to inflammation, fever, and sepsis.
- This is the PRIMARY mechanism responsible for the systemic inflammatory response (fever, confusion, hypotension) seen in Gram-negative sepsis.
*Incorrect: Nucleic acid*
- Bacterial nucleic acids (DNA, RNA) can be recognized by intracellular **Toll-like receptors** (e.g., TLR9 for unmethylated CpG DNA, TLR3 for dsRNA) during infection.
- However, nucleic acids are generally not the *primary* inducer of the systemic innate immune response (like fever and confusion attributed to sepsis) in intact Gram-negative bacterial infections; this role belongs to LPS.
*Incorrect: Secreted toxin*
- While many bacteria secrete **exotoxins** that can cause specific disease symptoms, the general systemic inflammatory response (fever, confusion, decreased urine output indicating sepsis) to Gram-negative bacteria is primarily mediated by **endotoxin (LPS)**, which is a component of the bacterial cell wall, not a secreted toxin.
- *Proteus mirabilis* does produce toxins like hemolysin and urease, but these are not the main driver of the systemic inflammatory response seen in sepsis.
*Incorrect: Polyribosylribitol phosphate*
- **Polyribosylribitol phosphate (PRP)** is a major virulence factor and component of the capsule of *Haemophilus influenzae* type b (Hib).
- It is a polysaccharide antigen recognized by the immune system, but it is not found in Gram-negative rods like *Proteus mirabilis* and is not the primary component responsible for the generalized innate immune response to Gram-negative bacteria.
*Incorrect: Teichoic acid in the cell wall*
- **Teichoic acids** (lipoteichoic acid and wall teichoic acid) are major components of the cell walls of **Gram-positive bacteria**.
- They are potent activators of the innate immune system (e.g., via TLR2), but they are *absent* in Gram-negative bacteria like the one described.
Question 8: A 46-year-old woman from Ecuador is admitted to the hospital because of tarry-black stools and epigastric pain for 2 weeks. The epigastric pain is relieved after meals, but worsens after 1–2 hours. She has no history of serious illness and takes no medications. Physical examination shows no abnormalities. Fecal occult blood test is positive. Esophagogastroduodenoscopy shows a bleeding duodenal ulcer. Microscopic examination of a duodenal biopsy specimen is most likely to show which of the following?
A. Dimorphic budding yeasts with pseudohyphae
B. Irregularly drumstick-shaped gram-positive rods
C. Gram-positive lancet-shaped diplococci
D. Curved, flagellated gram-negative rods (Correct Answer)
E. Teardrop-shaped multinucleated trophozoites
Explanation: ***Curved, flagellated gram-negative rods***
- The patient's symptoms (tarry stools, epigastric pain relieved by food but worsening 1-2 hours later) are highly suggestive of a **duodenal ulcer**. The EGD confirms a bleeding duodenal ulcer.
- The most common cause of duodenal ulcers worldwide is **Helicobacter pylori** infection, which is characterized microscopically as curved, flagellated gram-negative rods.
*Teardrop-shaped multinucleated trophozoites*
- This describes **Giardia lamblia**, which causes **giardiasis**, an intestinal infection leading to malabsorption, bloating, and diarrhea.
- While common in developing countries, it does not typically cause duodenal ulcers or upper GI bleeding.
*Dimorphic budding yeasts with pseudohyphae*
- This describes **Candida albicans**, a common cause of **esophagitis** in immunocompromised individuals.
- It does not typically cause duodenal ulcers, and its presence in the duodenum usually indicates severe immunocompromised status, which is not suggested in this healthy patient.
*Irregularly drumstick-shaped gram-positive rods*
- This morphology is characteristic of **Clostridium tetani** (which causes tetanus) or **Clostridium botulinum** (which causes botulism), due to their terminal spores creating a "drumstick" appearance.
- These organisms do not cause duodenal ulcers or gastrointestinal bleeding.
*Gram-positive lancet-shaped diplococci*
- This describes **Streptococcus pneumoniae**, which is a common cause of pneumonia, meningitis, and otitis media.
- It does not cause gastrointestinal infections or duodenal ulcers.
Question 9: A 4-year-old boy is brought to the physician because of a 3-day history of fever and left ear pain. Examination of the left ear shows a bulging tympanic membrane with green discharge. Gram stain of the discharge shows a gram-negative coccobacillus. The isolated organism grows on chocolate agar. The causal pathogen most likely produces a virulence factor that acts by which of the following mechanisms?
A. Inactivation of elongation factor
B. Binding of the Fc region of immunoglobulins
C. Inactivation of 60S ribosome
D. Cleavage of secretory immunoglobulins (Correct Answer)
E. Overactivation of adenylate cyclase
Explanation: ***Cleavage of secretory immunoglobulins***
- This scenario describes **acute otitis media** caused by *Haemophilus influenzae*, a gram-negative coccobacillus that grows on chocolate agar.
- *Haemophilus influenzae* produces **IgA protease**, which cleaves **secretory IgA** on mucosal surfaces, facilitating bacterial adherence and invasion.
*Inactivation of elongation factor*
- This mechanism is characteristic of **diphtheria toxin** (produced by *Corynebacterium diphtheriae*) and **Pseudomonas exotoxin A**, which block protein synthesis.
- While both can be serious pathogens, neither causes the typical otitis media described here, nor are they gram-negative coccobacilli.
*Binding of the Fc region of immunoglobulins*
- This is a virulence factor of **Protein A** produced by *Staphylococcus aureus*, which prevents opsonization and phagocytosis.
- *Staphylococcus aureus* is a gram-positive coccus, not a gram-negative coccobacillus, and its typical presentation in ear infections is different.
*Inactivation of 60S ribosome*
- This mechanism is associated with **Shiga toxin** (produced by *Shigella dysenteriae* and enterohemorrhagic *E. coli*) and **ricin**, leading to cell death.
- These pathogens typically cause gastrointestinal illness and are not commonly associated with otitis media in this manner.
*Overactivation of adenylate cyclase*
- This is characteristic of **cholera toxin** (produced by *Vibrio cholerae*) and **pertussis toxin** (produced by *Bordetella pertussis*), leading to increased cAMP levels and fluid secretion.
- While *Bordetella pertussis* is a gram-negative coccobacillus, it causes whooping cough, not acute otitis media with green discharge.
Question 10: A 24-year-old man comes to the physician with a 2-day history of fever, crampy abdominal pain, and blood-tinged diarrhea. He recently returned from a trip to Mexico. His temperature is 38.2°C (100.8°F). Abdominal examination shows diffuse tenderness to palpation; bowel sounds are hyperactive. Stool cultures grow nonlactose fermenting, oxidase-negative, gram-negative rods that do not produce hydrogen sulfide on triple sugar iron agar. Which of the following processes is most likely involved in the pathogenesis of this patient's condition?
A. Dissemination via bloodstream
B. Overactivation of adenylate cyclase
C. Flagella-mediated gut colonization
D. Invasion of colonic microfold cells
E. Inhibition of host cytoskeleton organization (Correct Answer)
Explanation: ***Inhibition of host cytoskeleton organization***
- The patient's symptoms (fever, crampy abdominal pain, blood-tinged diarrhea) and the microbiological findings (**nonlactose fermenting, oxidase-negative, gram-negative rods** that do not produce hydrogen sulfide) are characteristic of **Shigella infection**.
- **Shigella** invades colonic epithelial cells and manipulates the host cell's **actin cytoskeleton** through effector proteins (IpaA, IpaB, IpaC) delivered via a **Type III secretion system**.
- This cytoskeletal disruption enables **intracellular movement** via actin-based motility and **cell-to-cell spread**, allowing Shigella to evade immune defenses while causing characteristic inflammatory dysentery.
*Dissemination via bloodstream*
- While some bacterial infections cause bacteremia, **Shigella** infections are typically localized to the **gastrointestinal tract** and do not commonly disseminate systemically via the bloodstream.
- **Bacteremia** due to *Shigella* is rare and usually occurs only in immunocompromised individuals or young children with severe disease.
*Overactivation of adenylate cyclase*
- **Overactivation of adenylate cyclase** producing **cyclic AMP** and leading to **secretory diarrhea** is characteristic of toxins like **cholera toxin** or **heat-labile enterotoxin of E. coli**.
- **Shigella** primarily causes **inflammatory dysentery** through mucosal invasion and damage, not through this mechanism of fluid secretion.
*Flagella-mediated gut colonization*
- Many bacteria use **flagella** for motility and colonization, but **Shigella** species are notably **non-motile** and **lack flagella**.
- Their pathogenesis relies on invasion and intracellular spread rather than flagella-driven colonization.
*Invasion of colonic microfold cells*
- While **Shigella does initially invade through M cells (microfold cells)** in the colonic epithelium to gain entry into the lamina propria, this is just the **initial entry step**, not the primary pathogenic mechanism that causes disease.
- The key pathogenic process that leads to the characteristic symptoms is the **disruption of the host cytoskeleton** that enables intracellular replication and lateral spread through epithelial cells, causing the inflammatory dysentery seen in this patient.
