Coxiella burnetii — MCQs

10 questions

A medical technician is trying to isolate a pathogen from the sputum sample of a patient. The sample is heat fixed to a slide then covered with carbol fuchsin stain and heated again. After washing off the stain with clean water, the slide is covered with 1% sulfuric acid for decolorization. The sample is rinsed again and stained with methylene blue. Microscopic examination shows numerous red, branching filamentous organisms. Which of the following is the most likely isolated pathogen?

An investigator is studying the growth of an organism in different media. The organism is inoculated on a petri dish that contains heated sheep blood, vancomycin, nystatin, trimethoprim, and colistin. The resulting growth medium is incubated at 37°C. Numerous small, white colonies are seen after incubation for 48 hours. This organism is most likely to cause which of the following conditions?

A 27-year-old woman visits your office with a 3-day complaint of fever, malaise, myalgias, and headaches associated with vulvar itching, vulvar soreness, dysuria, and urethral discharge. The pain when urinating is so severe that she has attempted to avoid the use of the toilet. She just returned from a spring break in the tropics and explains that she had multiple unprotected sexual encounters with men and women. Upon physical examination, the temperature was 38.7°C (101.6°F) and the heart rate was 90/min. The pelvic examination revealed vulvar ulcers, vulvar excoriations, erythema, vaginal mucosa edema, and ulcerative cervicitis. Which of the following will best aid you in the identification of the specific organism causing the above symptoms?

A 45-year-old man presents to the physician with a complaint of recurrent chest pain for the last 2 weeks. He mentions that the pain is not present at rest, but when he walks for some distance, he feels pain like a pressure over the retrosternal region. The pain disappears within a few minutes of stopping the physical activity. He is an otherwise healthy man with no known medical disorder. He does not smoke or have any known dependence. There is no family history of ischemic heart disease or vascular disorder. On physical examination, his vital signs, including blood pressure, are normal. The diagnosis of coronary artery disease due to atherosclerosis is made. Which of the following is known to be an infection associated with this patient’s condition?

A previously healthy 24-year-old woman comes to the physician because of fever, headache, myalgia, photophobia, and a nonproductive cough for 2 days. Three weeks ago, she received a parrot as a birthday present. Her temperature is 38.5°C (101.3°F). Pulmonary examination shows crackles at both lung bases. Her leukocyte count is 8,000/mm3. An x-ray of the chest shows diffuse patchy infiltrates that are most prominent in the lower lobes. Which of the following is the most likely causal organism?

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 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 3-year-old female is found to have unusual susceptibility to infections by catalase-producing organisms. This patient likely has a problem with the function of which of the following cell types?

While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?

Q10

A 27-year-old man comes to the physician with throbbing right scrotal pain for 1 day. He has also had a burning sensation on urination during the last 4 days. He is sexually active with multiple female partners and does not use condoms. Physical examination shows a tender, palpable swelling on the upper pole of the right testicle; lifting the testicle relieves the pain. A Gram stain of urethral secretions shows numerous polymorphonuclear leukocytes but no organisms. Which of the following is the most likely causal pathogen of this patient's symptoms?

Coxiella burnetii US Medical PG Practice Questions and MCQs

Question 1: A medical technician is trying to isolate a pathogen from the sputum sample of a patient. The sample is heat fixed to a slide then covered with carbol fuchsin stain and heated again. After washing off the stain with clean water, the slide is covered with 1% sulfuric acid for decolorization. The sample is rinsed again and stained with methylene blue. Microscopic examination shows numerous red, branching filamentous organisms. Which of the following is the most likely isolated pathogen?

A. Cryptococcus neoformans
B. Tropheryma whipplei
C. Nocardia asteroides (Correct Answer)
D. Rickettsia rickettsii
E. Staphylococcus aureus

Explanation: ***Nocardia asteroides*** - The described staining procedure is a **modified acid-fast stain**, indicated by the use of **carbol fuchsin**, heating, and decolorization with **weak acid (1% sulfuric acid)**, followed by a counterstain with methylene blue. - **Nocardia species** are **weakly acid-fast bacteria** that resist decolorization with weak acids (1-3% sulfuric acid), appearing as **red, branching filamentous organisms** under this staining method. - The **modified acid-fast stain** uses weaker decolorizing agents compared to the standard Ziehl-Neelsen stain, making it suitable for detecting weakly acid-fast organisms like Nocardia. - Nocardia are aerobic actinomycetes commonly found in soil and can cause pulmonary infections, especially in immunocompromised patients. *Cryptococcus neoformans* - This is a **yeast** that is typically identified using an **India ink stain** to visualize its polysaccharide capsule, or through fungal stains like Gomori methenamine silver (GMS). - It would not appear as acid-fast red branching filaments with the described technique. *Tropheryma whipplei* - This bacterium is typically identified by **periodic acid-Schiff (PAS) stain** in tissue biopsies, which highlights its cell wall glycoproteins (appears magenta). - It is not acid-fast and would not retain the carbol fuchsin after acid decolorization. *Rickettsia rickettsii* - This is an **obligate intracellular bacterium** that is difficult to culture and is often diagnosed by **serological tests** or **immunohistochemistry** on skin biopsy specimens. - It is not acid-fast and would not be detected by this staining technique. *Staphylococcus aureus* - This is a **Gram-positive coccus** that would be stained **purple** by a Gram stain as it retains crystal violet. - It is not acid-fast and would be completely decolorized by sulfuric acid in the described procedure, appearing blue (counterstain color) rather than red.

Question 2: An investigator is studying the growth of an organism in different media. The organism is inoculated on a petri dish that contains heated sheep blood, vancomycin, nystatin, trimethoprim, and colistin. The resulting growth medium is incubated at 37°C. Numerous small, white colonies are seen after incubation for 48 hours. This organism is most likely to cause which of the following conditions?

A. Pontiac fever
B. Pseudomembranous colitis
C. Hemolytic uremic syndrome
D. Oral thrush
E. Gonorrhea (Correct Answer)

Explanation: ***Gonorrhea*** - The growth medium described is **Thayer-Martin agar**, a selective medium containing **heated sheep blood** (supplies NAD+), **vancomycin** (inhibits Gram-positives), **colistin** (inhibits Gram-negatives), **nystatin** (inhibits fungi), and **trimethoprim** (inhibits Proteus). This medium is specifically designed for the isolation of *Neisseria gonorrhoeae* from polymicrobial samples. - *Neisseria gonorrhoeae* typically grows as **small, translucent-to-white colonies** on selective media like Thayer-Martin agar, and incubation at 37°C in CO2 (not explicitly mentioned but often required) for 24-48 hours yields visible growth, causing **gonorrhea**. *Pontiac fever* - Pontiac fever is a mild, self-limiting form of **legionellosis**, caused by *Legionella pneumophila*. - *Legionella* requires a specialized medium such as **buffered charcoal yeast extract (BCYE) agar** for growth, not Thayer-Martin agar. *Pseudomembranous colitis* - This condition is caused by **toxin-producing *Clostridioides difficile***, often after antibiotic use. - *C. difficile* is an obligate anaerobe and requires **anaerobic conditions** and specific selective media (e.g., CCFA agar) for isolation, not Thayer-Martin agar under aerobic conditions. *Hemolytic uremic syndrome* - Hemolytic uremic syndrome (HUS) is often caused by **Shiga toxin-producing *Escherichia coli* (STEC)**, particularly O157:H7. - STEC can be isolated on media like **sorbitol MacConkey agar (SMAC)**, where O157:H7 appears as non-sorbitol fermenting colonies, distinct from the growth seen on Thayer-Martin. *Oral thrush* - Oral thrush is caused by *Candida albicans*, a yeast. - *Candida* would be inhibited by **nystatin** in the Thayer-Martin medium, which is an antifungal agent.

Question 3: A 27-year-old woman visits your office with a 3-day complaint of fever, malaise, myalgias, and headaches associated with vulvar itching, vulvar soreness, dysuria, and urethral discharge. The pain when urinating is so severe that she has attempted to avoid the use of the toilet. She just returned from a spring break in the tropics and explains that she had multiple unprotected sexual encounters with men and women. Upon physical examination, the temperature was 38.7°C (101.6°F) and the heart rate was 90/min. The pelvic examination revealed vulvar ulcers, vulvar excoriations, erythema, vaginal mucosa edema, and ulcerative cervicitis. Which of the following will best aid you in the identification of the specific organism causing the above symptoms?

A. Location of the lesions
B. Culture in Thayer-Martin agar
C. Direct fluorescence antigen (Correct Answer)
D. Serology
E. Tzanck smear

Explanation: ***Direct fluorescence antigen*** - A **direct fluorescence antigen (DFA)** test can rapidly identify viral antigens from the lesions, specifically for **Herpes Simplex Virus (HSV)**, which is highly suspected given the patient's symptoms (fever, malaise, myalgias, vulvar ulcers, dysuria, and recent unprotected sexual encounters). - **HSV** is a common cause of genital ulcers, and DFA offers a quick, sensitive, and specific method for detection directly from clinical samples. *Location of the lesions* - While the **location of the lesions** (vulvar ulcers) is characteristic of several sexually transmitted infections (STIs), it is not specific enough to identify the *specific organism* without further laboratory testing. - Conditions like syphilis, chancroid, and HSV all cause genital ulcers, making location alone insufficient for definitive diagnosis. *Culture in Thayer-Martin agar* - **Thayer-Martin agar** is selectively used for culturing **Neisseria gonorrhoeae**, which causes gonorrhea. - Although the patient has urethral discharge and dysuria, the presence of **vulvar ulcers** and systemic symptoms like fever and myalgias point away from uncomplicated gonorrhea. *Serology* - **Serology** detects antibodies to pathogens, indicating past or present infection, but is often less useful for identifying the *acute* causative organism in the initial stages of a symptomatic outbreak like this one. - For instance, HSV serology can distinguish between HSV-1 and HSV-2 exposure but does not confirm active infection in the way direct antigen methods or PCR do. *Tzanck smear* - A **Tzanck smear** can reveal characteristic **multinucleated giant cells** and **intranuclear inclusions**, which are indicative of herpesvirus infections (HSV or VZV). - However, it is less sensitive and specific than a direct fluorescence antigen test or PCR, and results can be variable depending on the quality of the smear and interpretation.

Question 4: A 45-year-old man presents to the physician with a complaint of recurrent chest pain for the last 2 weeks. He mentions that the pain is not present at rest, but when he walks for some distance, he feels pain like a pressure over the retrosternal region. The pain disappears within a few minutes of stopping the physical activity. He is an otherwise healthy man with no known medical disorder. He does not smoke or have any known dependence. There is no family history of ischemic heart disease or vascular disorder. On physical examination, his vital signs, including blood pressure, are normal. The diagnosis of coronary artery disease due to atherosclerosis is made. Which of the following is known to be an infection associated with this patient’s condition?

A. Coxiella burnetii
B. Rickettsia rickettsii
C. Chlamydophila pneumoniae (Correct Answer)
D. Mycoplasma pneumoniae
E. Legionella pneumophila

Explanation: ***Chlamydophila pneumoniae*** - *Chlamydophila pneumoniae* has been implicated in the pathogenesis of **atherosclerosis** and **coronary artery disease**. - **Chronic low-grade inflammation** induced by persistent *C. pneumoniae* infection is thought to contribute to plaque formation and instability. *Coxiella burnetii* - *Coxiella burnetii* is the causative agent of **Q fever**, which can manifest as **endocarditis** in its chronic form. - While *C. burnetii* can cause cardiovascular complications like endocarditis, it is not directly associated with the development of **atherosclerosis** itself. *Rickettsia rickettsii* - *Rickettsia rickettsii* causes **Rocky Mountain spotted fever**, a tick-borne illness. - This infection primarily causes widespread **vasculitis** and can lead to organ damage, but it is not a known risk factor for the development of chronic **atherosclerosis**. *Mycoplasma pneumoniae* - *Mycoplasma pneumoniae* is a common cause of **atypical pneumonia** and tracheobronchitis. - It is not recognized as a direct infectious agent contributing to the development of **atherosclerosis** or coronary artery disease. *Legionella pneumophila* - *Legionella pneumophila* causes **Legionnaires' disease**, a severe form of pneumonia. - While it can cause systemic inflammation during an acute infection, there is no established link between *L. pneumophila* and the long-term development of **atherosclerosis**.

Question 5: A previously healthy 24-year-old woman comes to the physician because of fever, headache, myalgia, photophobia, and a nonproductive cough for 2 days. Three weeks ago, she received a parrot as a birthday present. Her temperature is 38.5°C (101.3°F). Pulmonary examination shows crackles at both lung bases. Her leukocyte count is 8,000/mm3. An x-ray of the chest shows diffuse patchy infiltrates that are most prominent in the lower lobes. Which of the following is the most likely causal organism?

A. Cryptococcus neoformans
B. Francisella tularensis
C. Babesia microti
D. Chlamydia psittaci (Correct Answer)
E. Leptospira interrogans

Explanation: ***Chlamydia psittaci*** - The patient's recent acquisition of a **parrot** and subsequent development of **fever, headache, myalgia, photophobia, nonproductive cough**, and **diffuse patchy infiltrates** on CXR are highly characteristic of **psittacosis** (ornithosis), caused by *Chlamydia psittaci*. - This **atypical pneumonia** often presents with systemic symptoms out of proportion to respiratory findings and can mimic other viral illnesses. *Cryptococcus neoformans* - This is a **fungal infection** typically affecting immunocompromised individuals, causing **meningitis** or **pulmonary infections**. - While it can cause pneumonia, the direct link to a parrot and the specific constellation of symptoms point away from *Cryptococcus* in an otherwise healthy individual. *Francisella tularensis* - This bacterium causes **tularemia**, often associated with **rabbit or rodent exposure** or **tick bites**. - It presents with fever, ulceroglandular or pneumonic forms, but a history of parrot exposure makes it unlikely. *Babesia microti* - This is an **intraerythrocytic parasite** transmitted by **ticks**, causing **babesiosis**. - Symptoms include fever, hemolytic anemia, and fatigue, but it does not typically cause the described respiratory symptoms or diffuse patchy infiltrates. *Leptospira interrogans* - This spirochete causes **leptospirosis**, an infection associated with **exposure to animal urine** or contaminated water. - While it can present with fever, headache, and myalgia, the prominent respiratory symptoms and the specific parrot exposure do not fit well with leptospirosis, which more commonly involves liver and kidney dysfunction.

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

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.

Question 7: 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

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.

Question 8: A 3-year-old female is found to have unusual susceptibility to infections by catalase-producing organisms. This patient likely has a problem with the function of which of the following cell types?

A. Natural killer cells
B. Eosinophils
C. B cells
D. T cells
E. Neutrophils (Correct Answer)

Explanation: ***Neutrophils*** - Catalase-producing organisms, such as *Staphylococcus aureus* or *Aspergillus*, are typically cleared by **phagocytic cells**, specifically **neutrophils**, which use the **respiratory burst** to produce reactive oxygen species. - A defect in neutrophil function, particularly in the **NADPH oxidase enzyme complex** responsible for the respiratory burst, leads to **chronic granulomatous disease (CGD)**, characterized by increased susceptibility to infections by these specific pathogens. *Natural killer cells* - **Natural killer (NK) cells** are primarily involved in the anti-viral and anti-tumor immune responses, recognizing and **killing infected or malignant cells**. - They do not play a primary role in clearing bacterial or fungal infections, especially those caused by catalase-producing organisms. *Eosinophils* - **Eosinophils** are primarily involved in defense against **parasitic infections** and in mediating **allergic reactions**. - Their role in clearing common bacterial or fungal infections is limited. *B cells* - **B cells** are responsible for **humoral immunity**, producing **antibodies** that neutralize pathogens and toxins or opsonize them for phagocytosis. - While antibodies can aid in the clearance of many pathogens, a primary deficiency in B cell function (e.g., agammaglobulinemia) would lead to broad susceptibility to encapsulated bacteria, not specifically catalase-positive organisms. *T cells* - **T cells** are central to **cell-mediated immunity**, recognizing and eliminating intracellular pathogens or directly killing infected cells. - Deficiencies in T cell function (e.g., SCID) lead to severe immunodeficiency with susceptibility to opportunistic infections, but do not specifically point to problems with catalase-producing organisms as the hallmark.

Question 9: While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?

A. Quellung reaction (Correct Answer)
B. Hemolytic reaction when grown on sheep blood agar
C. Bile solubility
D. Optochin sensitivity
E. Motility

Explanation: ***Quellung reaction*** - The **Quellung reaction** tests for the presence of the **polysaccharide capsule**, which is the primary virulence factor of *S. pneumoniae*. - An **avirulent strain** that cannot cause disease would most likely lack the capsule and show a **negative Quellung reaction** (no capsular swelling), deviating from the **positive reaction** seen in typical encapsulated pathogenic strains. - The capsule enables *S. pneumoniae* to evade phagocytosis and complement-mediated killing, which is essential for establishing infection in the lungs. *Hemolytic reaction when grown on sheep blood agar* - Both virulent and avirulent strains of *S. pneumoniae* typically exhibit **alpha-hemolysis** (partial hemolysis, producing a greenish discoloration) on sheep blood agar due to the production of pneumolysin. - This characteristic does not differentiate between pathogenic and non-pathogenic strains in terms of disease-causing ability. *Bile solubility* - *S. pneumoniae* is characteristically **bile-soluble** due to the presence of autolysin enzymes that are activated by bile salts, leading to cellular lysis. - This property is a **species characteristic** present in both virulent and avirulent strains, thus it would not explain the inability to cause disease. *Optochin sensitivity* - *S. pneumoniae* is universally **sensitive to optochin**, a chemical agent that inhibits its growth and is used for laboratory identification. - This characteristic is used for **species identification** but does not correlate with strain virulence or disease-causing ability. *Motility* - *Streptococcus pneumoniae* is a **non-motile** bacterium; it lacks flagella. - This characteristic is consistent across all strains and is not a virulence factor for this species.

Question 10: A 27-year-old man comes to the physician with throbbing right scrotal pain for 1 day. He has also had a burning sensation on urination during the last 4 days. He is sexually active with multiple female partners and does not use condoms. Physical examination shows a tender, palpable swelling on the upper pole of the right testicle; lifting the testicle relieves the pain. A Gram stain of urethral secretions shows numerous polymorphonuclear leukocytes but no organisms. Which of the following is the most likely causal pathogen of this patient's symptoms?

A. Mycobacterium tuberculosis
B. Pseudomonas aeruginosa
C. Mumps virus
D. Chlamydia trachomatis (Correct Answer)
E. Staphylococcus aureus

Explanation: ***Chlamydia trachomatis*** - The patient's presentation with **epididymitis** (scrotal pain, tender palpable swelling on the upper pole of the testicle), **dysuria**, and a history of **multiple sexual partners without condoms** is classic for a sexually transmitted infection. - **Positive Prehn's sign** (pain relief with testicular elevation) supports epididymitis over testicular torsion. - The Gram stain showing **numerous polymorphonuclear leukocytes but no organisms** is highly suggestive of *C. trachomatis* infection, as it is an **obligate intracellular bacterium** that does not readily stain with Gram stain. - This finding distinguishes it from *Neisseria gonorrhoeae* (the other common cause of STI-related epididymitis in young men), which would appear as **Gram-negative intracellular diplococci**. *Mycobacterium tuberculosis* - **Tuberculosis epididymitis** is rare in developed countries and typically presents with a more **insidious onset** over weeks to months, not acute onset over 1 day. - It may involve caseating granulomas and is more common in immunocompromised patients. - It would not explain the acute dysuria or the Gram stain findings of PMNs without organisms in a patient with risk factors for common STIs. *Pseudomonas aeruginosa* - **Pseudomonas epididymitis** is typically seen in older men (>35 years), those with urinary tract abnormalities, or after instrumentation/catheterization. - It is uncommon in young, sexually active individuals without these risk factors. - Gram stain would show **Gram-negative rods**, which is inconsistent with the "no organisms" finding. *Mumps virus* - **Mumps orchitis** typically presents with **testicular pain and swelling** (affecting the testis itself, not the epididymis). - Mumps orchitis is usually preceded by **parotitis** (salivary gland swelling) 4-8 days earlier. - It does not typically cause dysuria or lead to urethral secretions with PMNs. - Mumps is now rare due to widespread MMR vaccination. *Staphylococcus aureus* - **Staphylococcus aureus** can cause epididymitis, especially in cases of direct trauma, hematogenous spread, or in patients with indwelling catheters. - However, it's not a common cause of sexually transmitted epididymitis in young, healthy men. - A Gram stain would reveal **Gram-positive cocci in clusters**, which was not seen in this case.

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