Legionella pneumophila US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Legionella pneumophila. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Legionella pneumophila US Medical PG Question 1: A 65-year-old woman is brought to the emergency department by her daughter for fever and cough. She just returned from a cruise trip to the Bahamas with her family 5 days ago and reports that she has been feeling ill since then. She endorses fever, productive cough, and general malaise. Her daughter also mentions that the patient has been having some diarrhea but reports that the rest of her family has been experiencing similar symptoms. Physical examination was significant for localized crackles at the right lower lobe. Laboratory findings are as follows:
Serum
Na+: 130 mEq/L
K+: 3.9 mEq/L
Cl-: 98 mEq/L
HCO3-: 27 mEq/L
Mg2+: 1.8 mEq/L
What findings would you expect in this patient?
- A. High titers of cold agglutinins
- B. Gram-negative rod on chocolate agar with factors V and X
- C. Gram-negative on silver stain (Correct Answer)
- D. Gram-positive diplococci on Gram stain
- E. Broad-based budding on fungal sputum culture
Legionella pneumophila Explanation: ***Gram-negative on silver stain***
- The patient's symptoms (fever, productive cough, malaise, diarrhea), recent cruise travel, and hyponatremia are classic presentations of **Legionnaires' disease** caused by *Legionella pneumophila*.
- *Legionella* is a **Gram-negative rod** that stains poorly with Gram stain and is best visualized using **silver stain**.
*High titers of cold agglutinins*
- **Cold agglutinins** are typically associated with **atypical pneumonia** caused by *Mycoplasma pneumoniae*.
- While *Mycoplasma* can cause similar respiratory symptoms, the presence of diarrhea and hyponatremia points away from it.
*Gram-negative rod on chocolate agar with factors V and X*
- This describes the growth requirements for *Haemophilus influenzae*, which needs **hematin (factor X)** and **NAD (factor V)** to grow on chocolate agar.
- While *H. influenzae* can cause pneumonia, the patient's specific presentation (cruise travel, diarrhea, hyponatremia) is more indicative of *Legionella*.
*Gram-positive diplococci on Gram stain*
- This microscopic finding is characteristic of **Streptococcus pneumoniae**, the most common cause of **community-acquired pneumonia**.
- Although *S. pneumoniae* can cause pneumonia, the detailed clinical picture, including hyponatremia and diarrhea, is not typical for uncomplicated pneumococcal pneumonia.
*Broad-based budding on fungal sputum culture*
- **Broad-based budding** is a characteristic feature of **Blastomyces dermatitidis**, a cause of fungal pneumonia.
- While fungal infections can cause pneumonia, the rapid onset, cruise exposure, and systemic symptoms (diarrhea, hyponatremia) are not classic for blastomycosis.
Legionella pneumophila US Medical PG Question 2: 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
Legionella pneumophila 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.
Legionella pneumophila US Medical PG Question 3: A 69-year-old man is brought to the emergency department by his wife because of fever, cough, diarrhea, and confusion for 2 days. He recently returned from a cruise to the Caribbean. He has a history of chronic obstructive pulmonary disease. He has smoked one pack of cigarettes daily for 40 years. His temperature is 39.1°C (102.4°F), pulse is 83/min, and blood pressure is 111/65 mm Hg. He is confused and oriented only to person. Physical examination shows coarse crackles throughout both lung fields. His serum sodium concentration is 125 mEq/L. Culture of the most likely causal organism would require which of the following mediums?
- A. Charcoal yeast extract agar (Correct Answer)
- B. Eosin-methylene blue agar
- C. Mannitol salt agar
- D. Chocolate agar
- E. Eaton agar
Legionella pneumophila Explanation: ***Charcoal yeast extract agar***
- The patient's symptoms (fever, cough, diarrhea, confusion, hyponatremia) and risk factors (COPD, smoking history, recent cruise travel) are highly suggestive of **Legionnaires' disease** caused by *Legionella pneumophila*.
- *Legionella* is a fastidious organism that requires **cysteine** and **iron salts** for growth, which are provided in **buffered charcoal yeast extract (BCYE) agar**.
*Eosin-methylene blue agar*
- This is a **selective and differential medium** used for the isolation and differentiation of **Gram-negative enteric bacteria**, particularly useful for identifying coliforms like *E. coli*.
- It contains dyes that inhibit Gram-positive bacteria and differentiate lactose fermenters, which is not relevant for *Legionella*.
*Mannitol salt agar*
- This is a **selective and differential medium** primarily used for the isolation and identification of **staphylococci**, especially *Staphylococcus aureus*.
- It contains a high salt concentration to inhibit most bacteria and mannitol to differentiate *S. aureus* (which ferments mannitol) from other staphylococci.
*Chocolate agar*
- This enriched medium is used for the isolation of fastidious bacteria such as **Haemophilus influenzae** and **Neisseria species**, which require factors like **hemin (X factor)** and **NAD (V factor)**.
- While it supports the growth of many pathogenic bacteria, it does not provide the specific growth requirements for *Legionella*.
*Eaton agar*
- This specialized medium is primarily used for the isolation and cultivation of **Mycoplasma pneumoniae**, a common cause of "walking pneumonia."
- *Mycoplasma pneumoniae* is a bacterium that lacks a cell wall and has unique growth requirements, distinct from *Legionella*.
Legionella pneumophila US Medical PG Question 4: A 62-year-old man is brought to the emergency department with fatigue, dry cough, and shortness of breath for 3 days. He reports a slight fever and has also had 3 episodes of watery diarrhea earlier that morning. Last week, he attended a business meeting at a hotel and notes some of his coworkers have also become sick. He has a history of hypertension and hyperlipidemia. He takes atorvastatin, hydrochlorothiazide, and lisinopril. He appears in mild distress. His temperature is 102.1°F (38.9°C), pulse is 56/min, respirations are 16/min, and blood pressure is 150/85 mm Hg. Diffuse crackles are heard in the thorax. Examination shows a soft and nontender abdomen. Laboratory studies show:
Hemoglobin 13.5 g/dL
Leukocyte count 15,000/mm3
Platelet count 130,000/mm3
Serum
Na+ 129 mEq/L
Cl- 100 mEq/L
K+ 4.6 mEq/L
HCO3- 22 mEq/L
Urea nitrogen 14 mg/dL
Creatinine 1.3 mg/dL
An x-ray of the chest shows infiltrates in both lungs. Which of the following is the most appropriate next step in diagnosis?
- A. Urine antigen assay (Correct Answer)
- B. CT Chest
- C. Direct immunofluorescent antibody test
- D. Stool culture
- E. Polymerase chain reaction
Legionella pneumophila Explanation: ***Urine antigen assay***
- This patient presents with **pneumonia symptoms** (low-grade fever, dry cough, dyspnea, bilateral infiltrates) along with **gastrointestinal symptoms** (watery diarrhea) and **hyponatremia**, after attending a hotel meeting with other sick attendees. These are classic features of **Legionnaires' disease**.
- A **urine antigen assay** is a rapid and highly specific test for **Legionella pneumophila serogroup 1**, which causes the majority of Legionnaires' disease cases.
*CT Chest*
- A CT scan of the chest would provide more detailed imaging of the lung infiltrates but is typically used to characterize findings once pneumonia is diagnosed or to rule out other lung pathologies, not as an initial diagnostic test for the specific pathogen.
- While it can reveal characteristic patterns, it doesn't identify the causative organism and is not the most appropriate *next step in diagnosis* for a presumed Legionella infection.
*Direct immunofluorescent antibody test*
- A **direct immunofluorescent antibody (DFA) test** is used to identify legionella in respiratory secretions. However, collecting a sufficiently good sputum sample can be difficult, especially with a **dry cough**.
- Its sensitivity is lower than urine antigen testing for serogroup 1 and requires a respiratory sample, making it less convenient for initial diagnosis.
*Stool culture*
- While the patient has diarrhea, a **stool culture** would primarily detect typical bacterial enteric pathogens (e.g., Salmonella, Shigella, Campylobacter) and would not identify **Legionella**.
- The diarrhea, in this context, is likely an extrapulmonary manifestation of Legionnaires' disease caused by Legionella, not a separate primary enteric infection.
*Polymerase chain reaction*
- **PCR testing** can detect Legionella DNA in respiratory samples, offering high sensitivity and specificity.
- However, it is generally less rapid and widely available than the urine antigen test for initial diagnosis of Legionella pneumophila serogroup 1, which is the most common cause of Legionnaires' disease.
Legionella pneumophila US Medical PG Question 5: A 41-year-old homeless man is brought to the emergency department complaining of severe fever, dizziness, and a persistent cough. The patient has a history of long-standing alcohol abuse and has frequently presented to the emergency department with acute alcohol intoxication. The patient states that his cough produces ‘dark brown stuff’ and he provided a sample for evaluation upon request. The patient denies having any other underlying medical conditions and states that he has no other symptoms. He denies taking any medications, although he states that he knows he has a sulfa allergy. On observation, the patient looks frail and severely fatigued. The vital signs include: blood pressure 102/72 mm Hg, pulse 98/min, respiratory rate 15/min, and temperature 37.1°C (98.8°F). Auscultation reveals crackles in the left upper lobe and chest X-ray reveals an infiltrate in the same area. Which of the following is the most appropriate treatment for this patient?
- A. Trimethoprim-sulfamethoxazole
- B. Vancomycin
- C. Clindamycin (Correct Answer)
- D. Piperacillin-tazobactam
- E. Ciprofloxacin
Legionella pneumophila Explanation: ***Clindamycin***
- The patient, a severely fatigued, homeless man with a history of alcohol abuse, presents with **fever, dizziness, cough producing 'dark brown stuff'**, and crackles/infiltrate in the left upper lobe. This clinical picture is highly suggestive of aspiration pneumonia and/or lung abscess, often caused by **anaerobic bacteria** found in the oral flora.
- **Clindamycin** is the **first-line treatment** for aspiration pneumonia in stable patients, with excellent activity against oral anaerobes, good lung tissue penetration, and proven efficacy in outpatient management.
*Trimethoprim-sulfamethoxazole*
- This antibiotic is **contraindicated** due to the patient's stated **sulfa allergy**.
- While effective against some respiratory pathogens, it does not provide adequate coverage for the likely anaerobic organisms involved in aspiration pneumonia.
*Vancomycin*
- Vancomycin is primarily used for serious **Gram-positive infections**, particularly **MRSA**, and does not provide adequate coverage for **anaerobic bacteria** typically involved in aspiration pneumonia.
- There is no clinical indication for MRSA coverage in this patient's presentation.
*Piperacillin-tazobactam*
- While piperacillin-tazobactam has excellent broad-spectrum activity, including against anaerobes, and would be effective for aspiration pneumonia, it is typically reserved for **hospitalized patients with severe pneumonia** or healthcare-associated infections.
- For a **stable outpatient** with aspiration pneumonia, clindamycin is preferred as it is more targeted, cost-effective, and the standard first-line therapy.
*Ciprofloxacin*
- Ciprofloxacin is a **fluoroquinolone** with good activity against many **Gram-negative bacteria** and some atypical respiratory pathogens.
- However, it has **poor activity against anaerobic bacteria**, which are the primary concern in aspiration pneumonia.
Legionella pneumophila US Medical PG Question 6: A 46-year-old man is brought to the emergency room by police after being found passed out on the sidewalk. He is intermittently alert and smells strongly of alcohol. He is unable to provide a history, but an electronic medical record search reveals that the patient has a history of alcohol abuse and was seen in the emergency room twice in the past year for alcohol intoxication. Further review of the medical record reveals that he works as a day laborer on a farm. His temperature is 98.8°F (37.1°C), blood pressure is 122/78 mmHg, pulse is 102/min, and respirations are 14/min. On examination, he is somnolent but arousable. He has vomitus on his shirt. He is given intravenous fluids and provided with supportive care. He vomits twice more and is discharged 6 hours later. However, 6 days after discharge, he presents to the emergency room again complaining of shortness of breath and fever. His temperature is 102°F (38.9°C), blood pressure is 100/58 mmHg, pulse is 116/min, and respirations are 24/min. The patient is actively coughing up foul-smelling purulent sputum. Which of the following is the most likely cause of this patient’s current symptoms?
- A. A: Prevotella melaninogenica (Correct Answer)
- B. D: Legionella pneumophila
- C. E: Mycoplasma pneumoniae
- D. C: Francisella tularensis
- E. B: Coxiella burnetii
Legionella pneumophila Explanation: **_Prevotella melaninogenica_**
- This patient's clinical presentation, including a history of **alcohol abuse**, vomiting, and subsequent development of **foul-smelling purulent sputum**, is highly suggestive of **aspiration pneumonia** caused by oral anaerobic bacteria.
- *Prevotella melaninogenica* is a common **anaerobic bacterium** found in the oral flora and is a frequent cause of aspiration pneumonia and lung abscesses, especially in patients with impaired consciousness due to alcohol.
*Coxiella burnetii*
- This is the causative agent of **Q fever**, which is often associated with contact with contaminated animal products or aerosols, particularly from livestock (e.g., cattle, sheep, goats).
- While the patient works on a farm, **foul-smelling purulent sputum** and the history of aspiration are not typical features of Q fever.
*Francisella tularensis*
- This bacterium causes **tularemia**, an infection typically acquired through contact with infected animals, insect bites (ticks), or contaminated water.
- The symptoms can include fever, chills, and pneumonia, but **foul-smelling sputum** is not a characteristic feature, and the mode of transmission doesn't align with the aspiration event.
*Legionella pneumophila*
- This bacterium causes **Legionnaires' disease**, a severe form of pneumonia often linked to contaminated water sources like air conditioning systems or hot tubs.
- While it causes pneumonia with fever and cough, **foul-smelling sputum** is not characteristic, and the patient's history of alcohol-induced aspiration is a stronger clue.
*Mycoplasma pneumoniae*
- This causes **"walking pneumonia,"** which is typically a milder respiratory infection with symptoms like gradual onset of cough, fever, and headache.
- It does not present with **foul-smelling purulent sputum** or a clinical picture consistent with aspiration pneumonia and its associated complications.
Legionella pneumophila US Medical PG Question 7: A 26-year-old man with HIV and a recent CD4+ count of 800 presents to his PCP with fever, cough, and dyspnea. He notes that he recently lost his job as a construction worker and has not been able to afford his HAART medication. His temperature is 102.6°F (39.2°C), pulse is 75/min, respirations are 24/min, and blood pressure is 135/92 mmHg. Physical exam reveals a tachypneic patient with scattered crackles in both lungs, and labs show a CD4+ count of 145 and an elevated LDH. The chest radiography is notable for bilateral diffuse interstitial infiltrates. For definitive diagnosis, the physician obtains a sputum sample. Which stain should he use to visualize the most likely responsible organism?
- A. Periodic acid schiff stain
- B. Silver stain (Correct Answer)
- C. Ziehl-Neelsen stain
- D. India ink stain
- E. Carbol fuchsin stain
Legionella pneumophila Explanation: ***Silver stain***
- The patient's presentation with **fever, cough, dyspnea, bilateral diffuse interstitial infiltrates**, and a **CD4+ count of 145** (indicating severe immunosuppression) is highly suggestive of ***Pneumocystis jirovecii*** **pneumonia (PCP)**, formerly known as ***Pneumocystis carinii***.
- ***Pneumocystis jirovecii*** **cysts** and **trophozoites** are best visualized using **silver-based stains** (e.g., Gomori methenamine silver stain) which stain the fungal cell walls dark brown or black.
*Periodic acid schiff stain*
- **PAS stain** is used to identify **glycogen, mucus, and fungal elements** like those of *Candida* or *Aspergillus*, by staining polysaccharides a magenta color.
- While it can stain some fungal organisms, it is **not the primary or most effective stain** for *Pneumocystis jirovecii*.
*Ziehl-Neelsen stain*
- The **Ziehl-Neelsen stain** (also known as acid-fast stain) is used to identify **acid-fast bacilli**, such as *Mycobacterium tuberculosis* or *Nocardia*.
- Although **tuberculosis** is common in HIV patients, the clinical presentation and CXR findings (diffuse interstitial infiltrates rather than cavitary lesions or granulomas) **do not strongly suggest tuberculosis** as the primary diagnosis here.
*India ink stain*
- The **India ink stain** is primarily used to visualize the **capsule of *Cryptococcus neoformans*** in cerebrospinal fluid or other body fluids, appearing as a halo around the yeast cells.
- This patient's symptoms are respiratory, and the likely pathogen is *Pneumocystis*, making India ink stain **inappropriate** for this suspected diagnosis.
*Carbol fuchsin stain*
- **Carbol fuchsin stain** is a component of the **acid-fast staining** procedure (like Ziehl-Neelsen), used as the primary stain to identify acid-fast organisms.
- As with Ziehl-Neelsen, this stain is for **mycobacteria** and would **not effectively visualize** ***Pneumocystis jirovecii***.
Legionella pneumophila US Medical PG Question 8: A 54-year-old woman comes to the physician because of lower back pain, night sweats, and a 5-kg (11-lb) weight loss during the past 4 weeks. She has rheumatoid arthritis treated with adalimumab. Her temperature is 38°C (100.4°F). Physical examination shows tenderness over the T10 and L1 spinous processes. Passive extension of the right hip causes pain in the right lower quadrant. The patient's symptoms are most likely caused by an organism with which of the following virulence factors?
- A. Proteins that bind to the Fc region of immunoglobulin G
- B. Protease that cleaves immunoglobulin A
- C. Polysaccharide capsule that prevents phagocytosis
- D. Surface glycolipids that prevent phagolysosome fusion (Correct Answer)
- E. Polypeptides that inactivate elongation factor 2
Legionella pneumophila Explanation: ***Surface glycolipids that prevent phagolysosome fusion***
- The patient's symptoms (low back pain, night sweats, weight loss, fever, spinal tenderness, and hip pain) in a patient on **adalimumab** (a TNF-alpha inhibitor) suggest **disseminated tuberculosis** (Pott disease).
- *Mycobacterium tuberculosis* uses **mycolic acids** and other surface glycolipids to prevent phagolysosome fusion, allowing it to survive and replicate within macrophages.
*Proteins that bind to the Fc region of immunoglobulin G*
- This virulence factor is characteristic of bacteria like *Staphylococcus aureus* (Protein A) and *Streptococcus pyogenes* (Protein G), which is not consistent with the clinical picture.
- These proteins interfere with opsonization and antibody-mediated immunity, but are not the primary mechanism of *Mycobacterium tuberculosis* survival within macrophages.
*Protease that cleaves immunoglobulin A*
- **IgA protease** is a virulence factor for bacteria such as *Neisseria gonorrhoeae*, *Neisseria meningitidis*, and *Streptococcus pneumoniae*, which colonize mucosal surfaces.
- This mechanism helps these bacteria evade mucosal immunity, but it is not relevant to the pathogenesis of tuberculosis.
*Polysaccharide capsule that prevents phagocytosis*
- A polysaccharide capsule is a major virulence factor for many encapsulated bacteria (e.g., *Streptococcus pneumoniae*, *Haemophilus influenzae*, *Neisseria meningitidis*) that helps them evade phagocytosis.
- However, *Mycobacterium tuberculosis* is not primarily characterized by a polysaccharide capsule for immune evasion; its internal survival within macrophages is more critical.
*Polypeptides that inactivate elongation factor 2*
- Toxins that inactivate **elongation factor 2** are associated with *Corynebacterium diphtheriae* (**diphtheria toxin**) and *Pseudomonas aeruginosa* (**exotoxin A**), leading to inhibition of protein synthesis.
- This mechanism is not involved in the pathogenesis of *Mycobacterium tuberculosis* infection or its ability to cause disseminated disease.
Legionella pneumophila US Medical PG Question 9: A 32-year-old man is brought to the physician by his wife for a 3-day history of fever, headaches, and myalgias. He returned from a camping trip in Oklahoma 10 days ago. He works as a computer salesman. His temperature is 38.1°C (100.6°F). Neurologic examination shows a sustained clonus of the right ankle following sudden passive dorsiflexion. He is disoriented to place and time but recognizes his wife. Laboratory studies show a leukocyte count of 1,700/mm3 and a platelet count of 46,000/mm3. A peripheral blood smear shows monocytes with intracytoplasmic morulae. Which of the following is the most likely causal organism?
- A. Coxiella burnetii
- B. Rickettsia rickettsii
- C. Anaplasma phagocytophilum
- D. Borrelia burgdorferi
- E. Ehrlichia chaffeensis (Correct Answer)
Legionella pneumophila Explanation: ***Correct: Ehrlichia chaffeensis***
- The presence of **intracytoplasmic morulae** in **monocytes** is a pathognomonic sign for *Ehrlichia chaffeensis* infection, which causes **human monocytic ehrlichiosis**.
- The patient's symptoms of **fever, headache, myalgias, thrombocytopenia, leukopenia**, and the history of a **camping trip** in an endemic area (Oklahoma) are highly consistent with ehrlichiosis.
*Incorrect: Coxiella burnetii*
- This bacterium causes **Q fever**, characterized by fever, headache, and atypical pneumonia, but it does **not cause intracytoplasmic morulae** in monocytes or frequently lead to the degree of leukopenia and thrombocytopenia seen here.
- While it can be acquired from environments, the **microscopic findings** rule it out in this specific case.
*Incorrect: Rickettsia rickettsii*
- This organism causes **Rocky Mountain spotted fever**, which presents with fever, headache, myalgias, and a characteristic **rash** that is usually present on the palms and soles, none of which are mentioned here.
- It does not form **intracytoplasmic morulae** in monocytes.
*Incorrect: Anaplasma phagocytophilum*
- This bacterium causes **human granulocytic anaplasmosis**, which is clinically similar to ehrlichiosis but forms **morulae in granulocytes** (neutrophils), not monocytes.
- The peripheral blood smear specifically identifies morulae in **monocytes**, directing towards *Ehrlichia*.
*Incorrect: Borrelia burgdorferi*
- This spirochete causes **Lyme disease**, characterized by an **expanding erythematous rash (erythema migrans)**, fever, and musculoskeletal symptoms, but it does not cause leukopenia or thrombocytopenia.
- It does not produce **morulae** in any blood cells.
Legionella pneumophila US Medical PG Question 10: A 27-year-old woman comes to the physician for a 1-week-history of painful urination and urinary frequency. She has no history of serious illness and takes no medications. She is sexually active with her boyfriend. Her temperature is 36.7°C (98.1°F). There is no costovertebral angle tenderness. Urine dipstick shows leukocyte esterase. A Gram stain does not show any organisms. Which of the following is the most likely causal pathogen?
- A. Escherichia coli
- B. Chlamydia trachomatis (Correct Answer)
- C. Gardnerella vaginalis
- D. Neisseria gonorrhoeae
- E. Trichomonas vaginalis
Legionella pneumophila Explanation: ***Chlamydia trachomatis***
- This patient presents with symptoms of **dysuria** and **urinary frequency**, consistent with a **urethritis**. The absence of bacteria on Gram stain points towards an **atypical pathogen**.
- **Chlamydia trachomatis** is a common cause of **non-gonococcal urethritis** and is a sexually transmitted infection, which fits with the sexually active history.
*Escherichia coli*
- **E. coli** is the most common cause of **bacterial urinary tract infections (UTIs)**, but a Gram stain in this case would typically reveal Gram-negative rods.
- While it causes dysuria and frequency, the **negative Gram stain** makes it less likely than an atypical pathogen.
*Gardnerella vaginalis*
- **Gardnerella vaginalis** is associated with **bacterial vaginosis**, causing a characteristic **fishy odor** and **vaginal discharge**, which are not reported here.
- It does not typically cause urethritis leading to painful urination and urinary frequency.
*Neisseria gonorrhoeae*
- **Neisseria gonorrhoeae** can cause **urethritis** with symptoms similar to those presented, and it is a sexually transmitted infection.
- However, Gram stain would typically show **Gram-negative diplococci** (intracellularly), which were not observed in this case.
*Trichomonas vaginalis*
- **Trichomonas vaginalis** is a **protozoan parasite** causing **trichomoniasis**, which commonly presents with **vaginitis** (frothy, green-yellow discharge, itching) or sometimes urethritis.
- While it is a **sexually transmitted infection**, this organism is not detected by Gram stain (which only stains bacteria); it would require **wet mount microscopy** for visualization. The primary presentation is usually vaginal, and it's less likely to be the sole cause of these urinary symptoms without other signs of vaginitis.
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