A 43-year-old man with a history of chronic alcoholism presents with a chronic cough and dyspnea. He says he traveled to Asia about 4 months ago and his symptoms started shortly after he returned. His temperature is 40.2°C (104.4°F) and pulse is 92/min. Physical examination reveals poor personal hygiene and a cough productive of foul blood-streaked sputum. Auscultation reveals decreased breath sounds on the right. A chest radiograph reveals an ill-defined circular lesion in the right middle lobe. Which of the following is true regarding this patient’s most likely diagnosis?
Q32
An 87-year-old woman is brought to the emergency department from her nursing home because of increasing confusion and lethargy for 12 hours. The nursing home aide says she did not want to get out of bed this morning and seemed less responsive than usual. She has Alzheimer's disease, hypertension, and a history of nephrolithiasis. She has chronic, intractable urinary incontinence, for which she has an indwelling urinary catheter. Current medications include galantamine, memantine, and ramipril. Her temperature is 38.5°C (101.3°F), pulse is 112/min, respiratory rate is 16/min, and blood pressure is 108/76 mm Hg. Physical examination shows mild tenderness to palpation of the lower abdomen. On mental status examination, she is oriented only to person. Laboratory studies show:
Hemoglobin 12.4 g/dL
Leukocyte count 9,000/mm3
Platelet count 355,000/mm3
Urine
pH 8.2
Glucose 1+
Protein 2+
Ketones negative
RBC 5/hpf
WBC 35/hpf
Bacteria moderate
Nitrites positive
Which of the following is the most likely causal organism?
Q33
A 22-year-old man comes to the physician because of an ulcer on his penis for 12 days. The ulcer is painful and draining yellow purulent material. He returned from a study abroad trip to India 3 months ago. His immunizations are up-to-date. He is sexually active with one female partner and uses condoms inconsistently. He appears uncomfortable. His temperature is 37.2°C (99.0°F), pulse is 94/min, and blood pressure is 120/80 mm Hg. Examination shows tender inguinal lymphadenopathy. There is a 2-cm ulcer with a necrotic base proximal to the glans of the penis. Which of the following is the most likely causal organism?
Q34
A 62-year-old man comes to the physician because of a 2-day history of fever, chills, and flank pain. Five days ago, he was catheterized for acute urinary retention. His temperature is 39.3°C (102.7°F). Physical examination shows right-sided costovertebral angle tenderness. Urine studies show numerous bacteria and WBC casts. Urine culture on blood agar grows mucoid, gray-white colonies. Urine culture on eosin methylene blue agar grows purple colonies with no metallic green sheen. Which of the following is the most likely causal pathogen?
Q35
An otherwise healthy 7-year-old boy is brought to the emergency department because of a 1-day history of involuntary muscle contractions and pain in his back and neck. Two weeks ago, he fell while playing in the sandbox and scraped both his knees. He has not received any vaccinations since birth. His temperature is 38.5°C (101.3°F). He is diaphoretic. Examination shows inability to open his mouth beyond 1 cm. There is hyperextension of the lumbar spine and resistance to neck flexion. Administration of which of the following would most likely have prevented this patient's current condition?
Q36
A 13-year-old boy is brought to the emergency department because of vomiting, diarrhea, abdominal pain, and dizziness for the past 3 hours with fever, chills, and muscle pain for the last day. He had presented 5 days ago for an episode of epistaxis caused by nasal picking and was treated with placement of anterior nasal packing. His parents report that the bleeding stopped, but they forgot to remove the nasal pack. His temperature is 40.0°C (104.0°F), pulse is 124/min, respirations are 28/min, and blood pressure is 96/68 mm Hg. He looks confused, and physical exam shows conjunctival and oropharyngeal hyperemia with a diffuse, erythematous, macular rash over the body that involves the palms and the soles. Removal of the anterior nasal pack shows hyperemia with purulent discharge from the underlying mucosa. Laboratory studies show:
Total white blood cell count 30,000/mm3 (30 x 109/L)
Differential count
Neutrophils 90%
Lymphocytes 8%
Monocytes 1%
Eosinophils 1%
Basophils 0%
Platelet count 95,000/mm3 (95 x 109/L)
Serum creatine phosphokinase 400 IU/L
What is the most likely diagnosis for this patient?
Q37
Antigen presentation of extracellular pathogens by antigen presenting cells requires endocytosis of the antigen, followed by the degradation in the acidic environment of the formed phagolysosome. Should the phagolysosome become unable to lower its pH, what is the most likely consequence?
Q38
A 15-year-old boy presents to the clinic complaining of an uncomfortable skin condition that started 2 years ago. The patient states that his skin feels ‘oily’ and that he is embarrassed by his appearance. On examination, he is a healthy-looking teenager who has reached the expected Tanner stage for his age. The skin on his face and back is erythematous and shows signs of inflammation. What is the microbiologic agent most associated with this presentation?
Q39
A 5-year-old girl is brought to the physician because of watery discharge from her right eye for 2 weeks. She and her parents, who are refugees from Sudan, arrived in Texas a month ago. Her immunization status is not known. She is at the 25th percentile for weight and the 50th percentile for height. Her temperature is 37.2°C (99°F), pulse is 90/min, and respirations are 18/min. Examination of the right eye shows matting of the eyelashes. Everting the right eyelid shows hyperemia, follicles, and papillae on the upper tarsal conjunctiva. Slit-lamp examination of the right eye shows follicles in the limbic region and the bulbar conjunctiva. There is corneal haziness with neovascularization at the 12 o'clock position. Examination of the left eye is unremarkable. Direct ophthalmoscopy of both eyes shows no abnormalities. Right pre-auricular lymphadenopathy is present. Which of the following is the most likely diagnosis in this patient?
Q40
An investigator is studying bacterial toxins in a nonpathogenic bacterial monoculture that has been inoculated with specific bacteriophages. These phages were previously cultured in a toxin-producing bacterial culture. After inoculation, a new toxin is isolated from the culture. Genetic sequencing shows that the bacteria have incorporated viral genetic information, including the gene for this toxin, into their genome. The described process is most likely responsible for acquired pathogenicity in which of the following bacteria?
Bacteria US Medical PG Practice Questions and MCQs
Question 31: A 43-year-old man with a history of chronic alcoholism presents with a chronic cough and dyspnea. He says he traveled to Asia about 4 months ago and his symptoms started shortly after he returned. His temperature is 40.2°C (104.4°F) and pulse is 92/min. Physical examination reveals poor personal hygiene and a cough productive of foul blood-streaked sputum. Auscultation reveals decreased breath sounds on the right. A chest radiograph reveals an ill-defined circular lesion in the right middle lobe. Which of the following is true regarding this patient’s most likely diagnosis?
A. A positive tuberculin test would be diagnostic of active infection.
B. DNA polymerase chain reaction (PCR) has poor sensitivity when applied to smear positive specimens.
C. Inoculation of a sputum sample into selective agar media needs to be incubated at 35–37°C (95.0–98.6°F) for up to 8 weeks. (Correct Answer)
D. Stains of gastric washing and urine have a high diagnostic yield on microscopy.
E. Ziehl-Neelsen staining is more sensitive than fluorescence microscopy with auramine-rhodamine stain.
Explanation: ***Inoculation of a sputum sample into selective agar media needs to be incubated at 35–37°C (95.0–98.6°F) for up to 8 weeks.***
- The patient's symptoms, travel history, imaging findings (ill-defined circular lesion), and social history (chronic alcoholism, poor hygiene) strongly suggest **pulmonary tuberculosis (TB)** with potential cavitation.
- **Mycobacterium tuberculosis** is a slow-growing organism; therefore, **culture on selective media** (e.g., Lowenstein-Jensen or Middlebrook media) for an extended period (typically 6-8 weeks) is the gold standard for diagnosis, allowing for identification and drug susceptibility testing.
*A positive tuberculin test would be diagnostic of active infection.*
- A **positive tuberculin skin test (TST)** or **interferon-gamma release assay (IGRA)** indicates **TB exposure** or **latent TB infection**, not necessarily active disease.
- Many individuals with latent TB will have a positive test but no active infection, requiring further diagnostic workup like sputum microscopy and culture to confirm active disease.
*DNA polymerase chain reaction (PCR) has poor sensitivity when applied to smear positive specimens.*
- **DNA PCR** is a rapid and highly **sensitive** and **specific** test for M. tuberculosis, especially valuable for **smear-positive specimens**.
- It can detect very small amounts of mycobacterial DNA, significantly reducing the diagnostic time compared to culture.
*Stains of gastric washing and urine have a high diagnostic yield on microscopy.*
- While M. tuberculosis can be found in gastric washings, especially in patients who cannot produce sputum, its **diagnostic yield for pulmonary TB by microscopy is not high** compared to sputum.
- **Urine microscopy** is rarely used for the diagnosis of pulmonary tuberculosis; it is only relevant in cases of **renal or genitourinary tuberculosis**.
*Ziehl-Neelsen staining is more sensitive than fluorescence microscopy with auramine-rhodamine stain.*
- **Fluorescence microscopy with auramine-rhodamine stain** is typically **more sensitive** than Ziehl-Neelsen staining for detecting acid-fast bacilli (AFB).
- The fluorescent stain allows for lower magnification scanning and better visualization of the bacilli, leading to a higher detection rate.
Question 32: An 87-year-old woman is brought to the emergency department from her nursing home because of increasing confusion and lethargy for 12 hours. The nursing home aide says she did not want to get out of bed this morning and seemed less responsive than usual. She has Alzheimer's disease, hypertension, and a history of nephrolithiasis. She has chronic, intractable urinary incontinence, for which she has an indwelling urinary catheter. Current medications include galantamine, memantine, and ramipril. Her temperature is 38.5°C (101.3°F), pulse is 112/min, respiratory rate is 16/min, and blood pressure is 108/76 mm Hg. Physical examination shows mild tenderness to palpation of the lower abdomen. On mental status examination, she is oriented only to person. Laboratory studies show:
Hemoglobin 12.4 g/dL
Leukocyte count 9,000/mm3
Platelet count 355,000/mm3
Urine
pH 8.2
Glucose 1+
Protein 2+
Ketones negative
RBC 5/hpf
WBC 35/hpf
Bacteria moderate
Nitrites positive
Which of the following is the most likely causal organism?
A. Enterococcus faecalis
B. Klebsiella pneumoniae
C. Staphylococcus saprophyticus
D. Escherichia coli
E. Proteus mirabilis (Correct Answer)
Explanation: ***Proteus mirabilis***
- The high urine pH (8.2), positive nitrites, and moderate bacteria, along with signs of infection in an elderly catheterized patient, are highly suggestive of a **urea-splitting organism**.
- **Proteus mirabilis** is a common cause of catheter-associated UTIs and produces urease, leading to alkaline urine and the formation of struvite stones, consistent with the patient's history of nephrolithiasis.
*Enterococcus faecalis*
- While *Enterococcus faecalis* can cause UTIs, it typically does not produce urease and therefore would not cause such a **markedly elevated urine pH** (above 7.5).
- Although it can cause positive nitrites, the absence of a strong alkali pH makes it less likely than *Proteus mirabilis*.
*Klebsiella pneumoniae*
- *Klebsiella pneumoniae* can cause UTIs and produce nitrites, but it is not typically a strong **urease producer** to the extent that would cause an alkaline urine pH of 8.2.
- It is more commonly associated with nosocomial infections, but the highly alkaline urine points away from it as the most likely cause here.
*Staphylococcus saprophyticus*
- *Staphylococcus saprophyticus* is a common cause of UTIs in young, sexually active women, but it is **rare in elderly, catheterized patients**.
- It is also not typically associated with such a high urine pH as seen in this case.
*Escherichia coli*
- *Escherichia coli* is the most common cause of UTIs, but it is a **non-urease-producing** bacterium and would typically result in acidic urine, or at least a less alkaline pH than 8.2.
- While it would cause positive nitrites and moderate bacteria, the elevated pH makes it less likely than *Proteus mirabilis* in this context.
Question 33: A 22-year-old man comes to the physician because of an ulcer on his penis for 12 days. The ulcer is painful and draining yellow purulent material. He returned from a study abroad trip to India 3 months ago. His immunizations are up-to-date. He is sexually active with one female partner and uses condoms inconsistently. He appears uncomfortable. His temperature is 37.2°C (99.0°F), pulse is 94/min, and blood pressure is 120/80 mm Hg. Examination shows tender inguinal lymphadenopathy. There is a 2-cm ulcer with a necrotic base proximal to the glans of the penis. Which of the following is the most likely causal organism?
A. Klebsiella granulomatis
B. Haemophilus ducreyi (Correct Answer)
C. Herpes simplex virus 2
D. Treponema pallidum
E. Chlamydia trachomatis
Explanation: ***Haemophilus ducreyi***
- The presentation of a **single, painful penile ulcer** with a **necrotic base** and **tender inguinal lymphadenopathy** is classic for **chancroid**,
- This condition is caused by **Haemophilus ducreyi**, and the patient's recent travel to India, where chancroid is endemic, increases the likelihood of this diagnosis.
*Klebsiella granulomatis*
- This bacterium causes **granuloma inguinale (donovanosis)**, which typically presents with **painless, beefy-red ulcers** that bleed easily,
- The ulcer described in the patient is **painful** and has a **necrotic base**, which is inconsistent with donovanosis.
*Herpes simplex virus 2*
- **Herpes simplex virus (HSV-2)** typically causes **multiple, painful vesicular lesions** that rupture to form shallow ulcers, often accompanied by systemic symptoms like fever and malaise.
- The patient describes a **single, large ulcer** with a necrotic base, which is not characteristic of herpetic lesions.
*Treponema pallidum*
- **Treponema pallidum** causes **syphilis**, which presents as a **painless chancre** with a clean base and firm, non-tender lymphadenopathy in its primary stage.
- The patient's ulcer is explicitly described as **painful** and draining **purulent material**, ruling out a syphilitic chancre.
*Chlamydia trachomatis*
- Certain serovars of **Chlamydia trachomatis** cause **lymphogranuloma venereum (LGV)**, which initially presents as a transient, **painless papule or ulcer** that often goes unnoticed, followed by significant, painful inguinal lymphadenopathy (buboes).
- While LGV involves painful lymphadenopathy, the initial ulcer is typically small and unnoticed, and the described ulcer is large, painful, and has a necrotic base, which is not characteristic of LGV.
Question 34: A 62-year-old man comes to the physician because of a 2-day history of fever, chills, and flank pain. Five days ago, he was catheterized for acute urinary retention. His temperature is 39.3°C (102.7°F). Physical examination shows right-sided costovertebral angle tenderness. Urine studies show numerous bacteria and WBC casts. Urine culture on blood agar grows mucoid, gray-white colonies. Urine culture on eosin methylene blue agar grows purple colonies with no metallic green sheen. Which of the following is the most likely causal pathogen?
A. Escherichia coli
B. Klebsiella pneumoniae (Correct Answer)
C. Pseudomonas aeruginosa
D. Proteus mirabilis
E. Staphylococcus saprophyticus
Explanation: ***Klebsiella pneumoniae***
- The presence of **mucoid, gray-white colonies** on blood agar and **purple colonies with no metallic green sheen** on EMB agar, along with a history of catheterization, fever, and flank pain strongly suggests *Klebsiella pneumoniae*.
- *Klebsiella* is a common cause of **catheter-associated UTIs** and often produces mucoid colonies due to its capsule.
*Escherichia coli*
- *E. coli* typically produces **metallic green sheen** on EMB agar due to rapid lactose fermentation, which is absent in this case.
- While *E. coli* is a common cause of UTIs, the specific culture findings differentiate it from *Klebsiella*.
*Pseudomonas aeruginosa*
- *Pseudomonas* often produces a **grape-like odor** and distinctive **blue-green pigment** on agar, neither of which is mentioned.
- It does not ferment lactose and would thus not produce purple colonies on EMB, but rather appear as colorless or clear colonies.
*Proteus mirabilis*
- *Proteus mirabilis* is known for its **swarming motility** on agar, which creates a characteristic spreading growth pattern, not merely mucoid colonies.
- It also produces **urease**, which can lead to alkaline urine and struvite stones, but the distinguishing colony morphology is not met.
*Staphylococcus saprophyticus*
- *Staphylococcus saprophyticus* is a **Gram-positive coccus** and would not grow purple colonies on EMB agar, which is selective for Gram-negative bacteria.
- It is a common cause of UTIs in young, sexually active women, which does not fit the patient's demographic.
Question 35: An otherwise healthy 7-year-old boy is brought to the emergency department because of a 1-day history of involuntary muscle contractions and pain in his back and neck. Two weeks ago, he fell while playing in the sandbox and scraped both his knees. He has not received any vaccinations since birth. His temperature is 38.5°C (101.3°F). He is diaphoretic. Examination shows inability to open his mouth beyond 1 cm. There is hyperextension of the lumbar spine and resistance to neck flexion. Administration of which of the following would most likely have prevented this patient's current condition?
A. Chemically-inactivated virus
B. Denatured bacterial product (Correct Answer)
C. Viable but weakened microorganism
D. Human immunoglobulin against a viral protein
E. Capsular polysaccharides
Explanation: ***Denatured bacterial product***
- This describes a **toxoid vaccine**, specifically the **tetanus toxoid vaccine**, which is a denatured form of the bacterial toxin.
- The patient's symptoms (trismus, back and neck pain, muscle contractions, **opisthotonus**) are classic for **tetanus**, and his unvaccinated status and recent wound increase his risk.
*Chemically-inactivated virus*
- This refers to an **inactivated viral vaccine**, which is effective against viral infections, not bacterial toxins.
- Tetanus is caused by a bacterial toxin, not a virus, making this vaccine type irrelevant to preventing the described condition.
*Viable but weakened microorganism*
- This describes a **live-attenuated vaccine**, which typically induces a strong immune response against the live pathogen itself.
- Tetanus prevention targets the toxin produced by *Clostridium tetani*, not the microorganism directly via an attenuated form.
*Human immunoglobulin against a viral protein*
- This describes **passive immunization** using antibodies against a viral protein, usually for viral infections or post-exposure prophylaxis.
- While passive immunization with tetanus immune globulin can *treat* tetanus, a vaccine is needed for *prevention*, and the target here is a bacterial toxin, not a viral protein.
*Capsular polysaccharides*
- This describes a component of **polysaccharide vaccines** used against encapsulated bacteria (e.g., *Streptococcus pneumoniae*, *Haemophilus influenzae type b*).
- *Clostridium tetani* is not an encapsulated bacterium, and its pathogenicity stems from its toxin, not its capsule.
Question 36: A 13-year-old boy is brought to the emergency department because of vomiting, diarrhea, abdominal pain, and dizziness for the past 3 hours with fever, chills, and muscle pain for the last day. He had presented 5 days ago for an episode of epistaxis caused by nasal picking and was treated with placement of anterior nasal packing. His parents report that the bleeding stopped, but they forgot to remove the nasal pack. His temperature is 40.0°C (104.0°F), pulse is 124/min, respirations are 28/min, and blood pressure is 96/68 mm Hg. He looks confused, and physical exam shows conjunctival and oropharyngeal hyperemia with a diffuse, erythematous, macular rash over the body that involves the palms and the soles. Removal of the anterior nasal pack shows hyperemia with purulent discharge from the underlying mucosa. Laboratory studies show:
Total white blood cell count 30,000/mm3 (30 x 109/L)
Differential count
Neutrophils 90%
Lymphocytes 8%
Monocytes 1%
Eosinophils 1%
Basophils 0%
Platelet count 95,000/mm3 (95 x 109/L)
Serum creatine phosphokinase 400 IU/L
What is the most likely diagnosis for this patient?
A. Stevens-Johnson syndrome
B. Measles
C. Disseminated gonococcal infection
D. Herpes simplex virus type 2 (HSV-2) meningitis
E. Toxic shock syndrome (Correct Answer)
Explanation: ***Toxic shock syndrome***
- The patient's presentation with **fever**, **hypotension**, **diffuse erythematous rash** involving palms/soles, **multisystem involvement** (vomiting, diarrhea, dizziness, confusion, elevated CPK), and the history of prolonged **nasal packing** (a common nidus for *Staphylococcus aureus* toxin production) is highly characteristic of **toxic shock syndrome (TSS)**.
- **Leukocytosis with neutrophilia** and **thrombocytopenia** are also common laboratory findings in TSS.
*Stevens-Johnson syndrome*
- Characterized by **mucocutaneous lesions** with epidermal detachment, forming **bullae** and **erosions**, often preceded by fever and flu-like symptoms.
- While it can involve mucous membranes, the **diffuse erythematous macular rash without bullae** and the rapid development of **shock** are not typical features.
*Measles*
- Presents with a **maculopapular rash** that typically starts on the face and spreads downwards, often coalescing. It is preceded by **prodromal symptoms** like cough, coryza, conjunctivitis, and **Koplik spots**.
- **Hypotension**, **severe multiorgan dysfunction**, and **nasal packing as a risk factor** are not features of measles.
*Disseminated gonococcal infection*
- Can cause **fever**, migratory **polyarthralgia**, and a **pustular or vesiculopustular rash** with hemorrhagic lesions, primarily on the extremities.
- The described **diffuse erythematous macular rash**, severe hypotension, and history of nasal packing do not fit the typical presentation of disseminated gonococcal infection.
*Herpes simplex virus type 2 (HSV-2) meningitis*
- Primarily causes **aseptic meningitis** with symptoms like fever, headache, stiff neck, and photophobia.
- It does not explain the **diffuse erythematous rash**, **hypotension**, **multisystem involvement**, or the role of **nasal packing** in the patient's presentation.
Question 37: Antigen presentation of extracellular pathogens by antigen presenting cells requires endocytosis of the antigen, followed by the degradation in the acidic environment of the formed phagolysosome. Should the phagolysosome become unable to lower its pH, what is the most likely consequence?
A. Deficient presentation of pathogens to CD4 T-cells (Correct Answer)
B. Deficient cell extravasation
C. Deficient presentation of pathogens to CD8 T-cells
D. Deficient NK cell activation
E. Deficient expression of B7
Explanation: ***Deficient presentation of pathogens to CD4 T-cells***
- The acidic environment of the **phagolysosome** is crucial for optimal **antigen degradation** and processing into peptides that can bind to **MHC class II molecules**.
- Without proper acidification, peptide loading onto **MHC class II** is impaired, leading to deficient presentation of extracellular pathogens to **CD4 T-cells**.
*Deficient cell extravasation*
- **Cell extravasation** involves events like rolling, adhesion, and transendothelial migration, which are primarily regulated by **adhesion molecules** and **chemokines**, not phagolysosomal pH.
- A defect in phagolysosomal pH would not directly impede the ability of cells to exit the vasculature.
*Deficient presentation of pathogens to CD8 T-cells*
- **CD8 T-cell** activation primarily involves the presentation of **intracellular antigens** via **MHC class I molecules**, which typically occurs through degradation in the **cytosol** via proteasomes.
- While some cross-presentation pathways exist, the primary mechanism of CD8 T-cell antigen presentation is not dependent on the acidification of phagolysosomes for extracellular pathogens.
*Deficient NK cell activation*
- **Natural Killer (NK) cells** recognize and kill target cells based on the presence or absence of **MHC class I molecules** and activating ligands, not on the processing of extracellular antigens within phagolysosomes.
- Their activation depends on cytokine environments and surface receptor interactions, not directly on phagolysosomal pH.
*Deficient expression of B7*
- **B7 molecules (CD80/CD86)** are **co-stimulatory molecules** expressed by antigen-presenting cells that are crucial for full T-cell activation. While antigen processing can influence APC activation, a specific defect in phagolysosomal pH would primarily affect the *presentation* of peptides, not the *expression* of co-stimulatory molecules.
- The expression of B7 is more broadly regulated by inflammatory signals and toll-like receptor (TLR) engagement, rather than being solely dependent on proper phagolysosomal acidification.
Question 38: A 15-year-old boy presents to the clinic complaining of an uncomfortable skin condition that started 2 years ago. The patient states that his skin feels ‘oily’ and that he is embarrassed by his appearance. On examination, he is a healthy-looking teenager who has reached the expected Tanner stage for his age. The skin on his face and back is erythematous and shows signs of inflammation. What is the microbiologic agent most associated with this presentation?
A. Human papillomavirus (HPV) strains 2 and 4
B. Cutibacterium acnes (Correct Answer)
C. Bartonella henselae
D. HHV-8
E. Streptococcus pyogenes
Explanation: ***Cutibacterium acnes***
- The presentation of "oily" skin, erythematous, inflamed skin on the face and back in a 15-year-old boy is highly suggestive of **acne vulgaris**.
- **Cutibacterium acnes** (formerly *Propionibacterium acnes*) is a commensal bacterium that thrives in the anaerobic, lipid-rich environment of the pilosebaceous unit, and its proliferation contributes significantly to the inflammatory response seen in acne.
*Human papillomavirus (HPV) strains 2 and 4*
- HPV strains 2 and 4 are typically associated with **cutaneous warts**, which manifest as benign skin growths and are not characterized by widespread oily, erythematous, and inflamed skin.
- The clinical picture of acne, characterized by **comedones, papules, pustules, and sometimes cysts**, is distinct from the lesions caused by HPV.
*Bartonella henselae*
- This bacterium is the causative agent of **cat-scratch disease**, which typically presents with localized lymphadenopathy following a cat scratch or bite.
- While it can manifest with unusual skin lesions (e.g., bacillary angiomatosis in immunocompromised individuals), it does not cause generalized oily, inflammatory acne.
*HHV-8*
- Human Herpesvirus 8 (HHV-8) is primarily associated with **Kaposi's sarcoma**, a vascular malignancy characterized by reddish-purple skin lesions, most commonly in immunocompromised individuals.
- It does not cause acne vulgaris or the described skin changes.
*Streptococcus pyogenes*
- *Streptococcus pyogenes* is known for causing a variety of skin infections such as **impetigo, cellulitis, and erysipelas**, which are acute bacterial infections.
- These infections typically present with rapidly developing, painful, and often pustular or vesicular lesions, distinct from the chronic, oily, inflammatory papules and pustules of acne.
Question 39: A 5-year-old girl is brought to the physician because of watery discharge from her right eye for 2 weeks. She and her parents, who are refugees from Sudan, arrived in Texas a month ago. Her immunization status is not known. She is at the 25th percentile for weight and the 50th percentile for height. Her temperature is 37.2°C (99°F), pulse is 90/min, and respirations are 18/min. Examination of the right eye shows matting of the eyelashes. Everting the right eyelid shows hyperemia, follicles, and papillae on the upper tarsal conjunctiva. Slit-lamp examination of the right eye shows follicles in the limbic region and the bulbar conjunctiva. There is corneal haziness with neovascularization at the 12 o'clock position. Examination of the left eye is unremarkable. Direct ophthalmoscopy of both eyes shows no abnormalities. Right pre-auricular lymphadenopathy is present. Which of the following is the most likely diagnosis in this patient?
A. Neisserial conjunctivitis
B. Trachoma conjunctivitis (Correct Answer)
C. Acute herpetic conjunctivitis
D. Angular conjunctivitis
E. Acute hemorrhagic conjunctivitis
Explanation: ***Trachoma conjunctivitis***
- The constellation of **follicles and papillae on the upper tarsal conjunctiva**, **limbal follicles**, **corneal haziness with neovascularization (pannus)**, and **pre-auricular lymphadenopathy** in a child from an endemic region (Sudan) is classic for **trachoma**.
- This chronic form of conjunctivitis is caused by *Chlamydia trachomatis* serovars A, B, and C, leading to progressive scarring that can eventually cause **trichiasis** and blindness.
*Neisserial conjunctivitis*
- This condition typically presents with **hyperacute onset**, **copious purulent discharge**, and significant eyelid swelling, often within days of birth or infection.
- While it can cause corneal involvement, the chronic follicular and papillary changes with limbal follicles and pannus are not characteristic.
*Acute herpetic conjunctivitis*
- Usually presents with **unilateral follicular conjunctivitis**, often accompanied by **periorbital vesicles** or a history of cold sores.
- While it can cause corneal involvement (typically **dendritic ulcers**), the specific follicular changes, presence of papillae, and chronic course leading to pannus seen here are not typical.
*Angular conjunctivitis*
- Characterized by **redness, excoriation, and maceration** primarily localized to the **outer canthus** (angle) of the eye, often caused by *Moraxella lacunata* or *Staphylococcus aureus*.
- It does not present with the diffuse follicular and papillary changes, limbal follicles, or corneal neovascularization described in this patient.
*Acute hemorrhagic conjunctivitis*
- This is typically an **acute, highly contagious viral conjunctivitis** characterized by **subconjunctival hemorrhages**, rapid onset, and usually resolves spontaneously.
- It does not cause chronic follicular changes, limbal follicles, or corneal neovascularization, and the duration in this patient (2 weeks) suggests a more chronic process.
Question 40: An investigator is studying bacterial toxins in a nonpathogenic bacterial monoculture that has been inoculated with specific bacteriophages. These phages were previously cultured in a toxin-producing bacterial culture. After inoculation, a new toxin is isolated from the culture. Genetic sequencing shows that the bacteria have incorporated viral genetic information, including the gene for this toxin, into their genome. The described process is most likely responsible for acquired pathogenicity in which of the following bacteria?
A. Staphylococcus aureus
B. Haemophilus influenzae
C. Neisseria meningitidis
D. Streptococcus pneumoniae
E. Corynebacterium diphtheriae (Correct Answer)
Explanation: ***Corynebacterium diphtheriae***
- The process described, where a bacterium acquires new genetic information (e.g., a toxin gene) from a bacteriophage, is called **lysogenic conversion** or **phage conversion**. *Corynebacterium diphtheriae* is the **classic example** of this mechanism, acquiring its toxigenicity through phage-mediated transfer of the **diphtheria toxin gene (tox gene)** via bacteriophage β.
- The diphtheria toxin is an **AB toxin** that ADP-ribosylates and thereby inactivates **elongation factor 2 (EF-2)**, inhibiting host cell protein synthesis and leading to the characteristic symptoms of diphtheria.
- This is the **prototypical and most clinically significant example** of lysogenic conversion in medical microbiology.
*Staphylococcus aureus*
- While *Staphylococcus aureus* can acquire some virulence factors via bacteriophages (e.g., **Panton-Valentine leukocidin**, some enterotoxins), many of its toxins are encoded on **mobile genetic elements** such as plasmids, pathogenicity islands, or chromosomal genes.
- However, *S. aureus* is **not the classic example** of lysogenic conversion described in this scenario. *C. diphtheriae* better exemplifies the acquisition of a major toxin exclusively through phage conversion.
*Haemophilus influenzae*
- *Haemophilus influenzae* primarily causes disease through its **polysaccharide capsule** (especially type b) and is a common cause of respiratory infections and meningitis.
- Its major virulence factors are typically chromosomally encoded or acquired through **transformation** (uptake of naked DNA), not through phage conversion for a primary toxin.
*Neisseria meningitidis*
- *Neisseria meningitidis* causes meningococcal disease, primarily due to its **polysaccharide capsule** and **endotoxin (LPS)**.
- While genetic exchange can occur, the acquisition of a major toxin gene by phage conversion as described is not a primary mechanism for its key virulence factors.
*Streptococcus pneumoniae*
- *Streptococcus pneumoniae* is a leading cause of pneumonia, meningitis, and otitis media, with its main virulence factor being its **polysaccharide capsule**.
- It primarily acquires genetic material through **transformation** (competence-mediated uptake of naked DNA), which contributes to antibiotic resistance and capsule types, but lysogenic conversion with toxin acquisition is not typical for its major virulence factors.
