Rickettsia species US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Rickettsia species. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Rickettsia species US Medical PG Question 1: A 45-year-old man presents to the physician with complaints of fever with rigors, headache, malaise, muscle pains, nausea, vomiting, and decreased appetite for the past 3 days. He informs the physician that he had been backpacking on the Appalachian Trail in the woods of Georgia in the month of June, 2 weeks ago, and had been bitten by a tick there. His temperature is 39.0°C (102.3°F), pulse is 94/min, respirations are 18/min, and blood pressure is 126/82 mm Hg. His physical exam does not reveal any significant abnormality except for mild splenomegaly. Laboratory studies show:
Total white blood cell count 3,700/mm3 (3.7 x 109/L)
Differential count
Neutrophils 85%
Lymphocytes 12%
Monocytes 2%
Eosinophils 1%
Basophils 0%
Platelet count 88,000/mm3 (95 x 109/L)
Serum alanine aminotransferase 140 IU/L
Serum aspartate aminotransferase 80 IU/L
Microscopic examination of a peripheral blood smear stained with Wright-Giemsa stain shows the presence of morulae in the cytoplasm of leukocytes. In addition to drugs for symptomatic relief, what is the most appropriate initial step in the treatment of this patient?
- A. Doxycycline (Correct Answer)
- B. Ceftriaxone
- C. Rifampin
- D. Ciprofloxacin
- E. Daptomycin
Rickettsia species Explanation: ***Doxycycline***
- The patient's presentation with **fever, myalgia, headache, thrombocytopenia, leukopenia, elevated liver enzymes**, a history of **tick bite** in an endemic area (Appalachian Trail, Georgia), and the presence of **morulae in leukocytes** strongly indicates **ehrlichiosis** or **anaplasmosis**.
- **Doxycycline** is the **first-line treatment** for all rickettsial diseases, including ehrlichiosis and anaplasmosis, regardless of age or pregnancy status, due to its effectiveness in preventing severe complications.
*Ceftriaxone*
- While effective against many bacterial infections, **ceftriaxone** is **not effective** against intracellular bacteria like *Ehrlichia* or *Anaplasma*.
- It is typically used for conditions like Lyme disease (later stages), meningitis, or community-acquired pneumonia, which do not match the complete clinical picture here.
*Rifampin*
- **Rifampin** is sometimes used for treatment of ehrlichiosis in patients who **cannot tolerate doxycycline**, but it is **not the first-line agent**.
- Its primary uses are for tuberculosis and prophylaxis of meningococcal disease, making it an inappropriate initial choice given the classic presentation.
*Ciprofloxacin*
- **Ciprofloxacin** is a fluoroquinolone antibiotic primarily used for urinary tract infections, respiratory infections, and some gastrointestinal infections.
- It has **no significant activity** against species of *Ehrlichia* or *Anaplasma* and is therefore not indicated for this condition.
*Daptomycin*
- **Daptomycin** is a lipopeptide antibiotic used for treating **Gram-positive bacterial infections**, especially those resistant to other antibiotics (e.g., MRSA).
- It is **ineffective** against the intracellular Gram-negative bacteria causing ehrlichiosis or anaplasmosis.
Rickettsia species US Medical PG Question 2: A 33-year-old man presents to the emergency department with a fever and fatigue. He states that he has not felt well since he returned from a hiking trip in Alabama. He is generally healthy and has no other medical conditions. His temperature is 101°F (38.3°C), blood pressure is 127/85 mmHg, pulse is 108/min, respirations are 14/min, and oxygen saturation is 99% on room air. Physical exam including a full dermatologic inspection is unremarkable. Laboratory studies are ordered as seen below.
Hemoglobin: 13 g/dL
Hematocrit: 39%
Leukocyte count: 2,200/mm^3 with normal differential
Platelet count: 77,000/mm^3
Serum:
Na+: 139 mEq/L
Cl-: 100 mEq/L
K+: 4.3 mEq/L
HCO3-: 24 mEq/L
BUN: 19 mg/dL
Glucose: 98 mg/dL
Creatinine: 1.3 mg/dL
Ca2+: 10.2 mg/dL
AST: 92 U/L
ALT: 100 U/L
Which of the following is the most likely diagnosis?
- A. Lyme disease
- B. Babesiosis
- C. Influenza
- D. Ehrlichiosis (Correct Answer)
- E. Rocky Mountain spotted fever
Rickettsia species Explanation: ***Ehrlichiosis***
- This patient's symptoms (fever, fatigue), recent travel to an **endemic area** (Alabama), **leukopenia** (WBC 2,200/mm^3), **thrombocytopenia** (platelet 77,000/mm^3), and **elevated liver enzymes** (AST 92, ALT 100) are highly characteristic of ehrlichiosis, a **tick-borne disease**.
- The absence of a rash helps differentiate it from some other tick-borne illnesses.
*Lyme disease*
- While Lyme disease is also tick-borne, it typically presents with an **erythema migrans rash**, which is absent in this case.
- Lyme disease is less commonly associated with the **pronounced leukopenia** and **thrombocytopenia** seen here.
*Rocky Mountain spotted fever*
- Rocky Mountain Spotted Fever (RMSF) is characterized by a **maculopapular rash** that often starts on the ankles and wrists and spreads centrally, involving the palms and soles. This rash is absent in the patient.
- While RMSF can cause thrombocytopenia and elevated liver enzymes, the **lack of rash is a key differentiator**.
*Babesiosis*
- Babesiosis is a tick-borne parasitic infection that causes **hemolytic anemia**, which is not clearly indicated by the patient's hemoglobin and hematocrit, and typically results in severe fatigue and sometimes splenomegaly.
- This condition is often seen in immunocompromised individuals or those without a spleen, and the labs here are more consistent with ehrlichiosis than babesiosis.
*Influenza*
- Influenza presents with fever, fatigue, myalgia, and respiratory symptoms, but it does not cause **thrombocytopenia**, **leukopenia**, or **elevated liver enzymes** to the extent seen in this patient.
- The symptoms are more indicative of a **tick-borne illness** given the travel history and specific lab abnormalities.
Rickettsia species US Medical PG Question 3: A 13-year-old girl is brought to the physician because of worsening fever, headache, photophobia, and nausea for 2 days. One week ago, she returned from summer camp. She has received all age-appropriate immunizations. Her temperature is 39.1°C (102.3°F). She is oriented to person, place, and time. Physical examination shows a maculopapular rash. There is rigidity of the neck; forced flexion of the neck results in involuntary flexion of the knees and hips. Cerebrospinal fluid studies show:
Opening pressure 120 mm H2O
Appearance Clear
Protein 47 mg/dL
Glucose 68 mg/dL
White cell count 280/mm3
Segmented neutrophils 15%
Lymphocytes 85%
Which of the following is the most likely causal organism?
- A. Echovirus (Correct Answer)
- B. Listeria monocytogenes
- C. Streptococcus pneumoniae
- D. Herpes simplex virus
- E. Neisseria meningitidis
Rickettsia species Explanation: ***Echovirus***
- The patient's symptoms (fever, headache, photophobia, maculopapular rash, neck rigidity) along with CSF findings of **lymphocytic pleocytosis**, **normal glucose**, and **moderately elevated protein** are highly suggestive of **aseptic meningitis**.
- **Enteroviruses**, such as Echovirus, are the most common cause of **viral (aseptic) meningitis**, especially in children and during summer months, fitting the patient's age and recent summer camp attendance.
*Listeria monocytogenes*
- This organism typically causes meningitis in **neonates, elderly, or immunocompromised individuals**, which does not fit this healthy 13-year-old girl.
- While it can cause lymphocytic pleocytosis, it is less likely given the patient's age and presentation.
*Streptococcus pneumoniae*
- This is a common cause of **bacterial meningitis**, characterized by **PMN predominance (neutrophilic pleocytosis)**, **low CSF glucose**, and **markedly elevated CSF protein**, which are not seen in this case.
- The patient is also described as having received all age-appropriate immunizations, likely including the pneumococcal vaccine.
*Herpes simplex virus*
- HSV can cause aseptic meningitis or encephalitis, but it often presents with **focal neurological deficits** or **seizures** in cases of encephalitis, which are absent here.
- While it can cause lymphocytic pleocytosis, the maculopapular rash is less typical for HSV meningitis compared to enteroviruses.
*Neisseria meningitidis*
- This typically causes **bacterial meningitis** with characteristic CSF findings of **neutrophilic pleocytosis**, **low glucose**, and **high protein**.
- Although it can cause a rash (petechial or purpuric), the CSF profile and absence of petechiae make bacterial meningitis less likely.
Rickettsia species US Medical PG Question 4: A previously healthy 5-year-old boy is brought to the physician because of increasing weakness and a retroauricular rash that started 2 days ago. The rash spread rapidly and involves the trunk and extremities. Last week, he had a mild sore throat, pink eyes, and a headache. His family recently immigrated from Ethiopia. His immunization status is unknown. The patient appears severely ill. His temperature is 38.5°C (101.3°F). Examination shows tender postauricular and suboccipital lymphadenopathy. There is a nonconfluent, maculopapular rash over the torso and extremities. Infection with which of the following is the most likely cause of this patient's symptoms?
- A. Togavirus (Correct Answer)
- B. Human herpesvirus 6
- C. Parvovirus
- D. Varicella zoster virus
- E. Paramyxovirus
Rickettsia species Explanation: ***Togavirus***
- This patient's presentation is classic for **rubella** (German measles), caused by the **rubella virus**, a **togavirus**.
- The hallmark clinical finding is **tender postauricular and suboccipital lymphadenopathy**, which appears before the rash and is pathognomonic for rubella.
- The **maculopapular rash** begins on the face (retroauricular region) and spreads cephalocaudally to the trunk and extremities over 2-3 days.
- The prodrome includes **mild symptoms** (low-grade fever, sore throat, mild conjunctivitis, headache), which is characteristic of rubella.
- The patient's **unknown immunization status** and immigration from a region with lower vaccination coverage increases the likelihood of rubella infection.
*Paramyxovirus*
- **Measles virus** is a paramyxovirus that causes rubeola, but the clinical presentation differs significantly from this case.
- Measles typically presents with the **"3 Cs"**: severe **cough**, **coryza** (profuse nasal discharge), and **conjunctivitis** (more prominent than rubella).
- **Koplik spots** (white spots on buccal mucosa) are pathognomonic for measles and appear before the rash.
- Measles causes **higher fever** (often >40°C) and more severe systemic illness than described here.
- While measles can have lymphadenopathy, the **prominent postauricular and suboccipital nodes are characteristic of rubella, not measles**.
*Human herpesvirus 6*
- **HHV-6** causes **roseola infantum** (exanthem subitum), typically in infants 6-24 months old.
- The classic presentation is **high fever for 3-5 days** that suddenly resolves, followed immediately by a rash (**"fever then rash"**).
- This patient had prodromal symptoms followed by rash while still febrile, which does not fit roseola.
- Roseola does not cause significant lymphadenopathy or conjunctivitis.
*Parvovirus*
- **Parvovirus B19** causes **erythema infectiosum** (fifth disease), characterized by a **"slapped cheek"** facial erythema followed by a reticular (lacy) rash on the trunk and extremities.
- The rash pattern and prominent lymphadenopathy in this case are not consistent with fifth disease.
- Fifth disease typically causes mild or no fever and lacks the retroauricular distribution seen here.
*Varicella zoster virus*
- **VZV** causes **chickenpox**, which presents with a **pruritic, vesicular rash** that appears in successive crops and progresses through stages (macule → papule → vesicle → crust).
- This patient has a **maculopapular, nonconfluent rash** without vesicles, which is inconsistent with chickenpox.
- Chickenpox does not typically cause prominent postauricular lymphadenopathy.
Rickettsia species US Medical PG Question 5: A 17-year-old boy comes to the physician because of fever, fatigue, and a sore throat for 12 days. He was prescribed amoxicillin at another clinic and now has a diffuse rash all over his body. He was treated for gonorrhea one year ago. He has multiple sexual partners and uses condoms inconsistently. He appears lethargic and thin. His BMI is 19.0 kg/m2. His temperature is 38.4°C (101.1°F), pulse 94/min, blood pressure 106/72 mm Hg. Examination shows a morbilliform rash over his extremities. Oropharyngeal examination shows tonsillar enlargement and erythema with exudates. Tender cervical and inguinal lymphadenopathy is present. Abdominal examination shows mild splenomegaly. Laboratory studies show:
Hemoglobin 14 g/dL
Leukocyte count 13,200/mm3
Platelet count 160,000/mm3
Which of the following is the next best step in management?
- A. Heterophile agglutination test (Correct Answer)
- B. ELISA for HIV
- C. Flow cytometry
- D. Anti-CMV IgM
- E. Throat swab culture
Rickettsia species Explanation: ***Heterophile agglutination test***
- The patient's symptoms (fever, fatigue, sore throat, generalized lymphadenopathy, splenomegaly, and **diffuse rash after amoxicillin exposure**) are highly suggestive of **infectious mononucleosis** caused by Epstein-Barr virus (EBV).
- A **heterophile agglutination test (Monospot test)** is the most appropriate initial diagnostic step for suspected infectious mononucleosis.
- The **amoxicillin-induced morbilliform rash** is a pathognomonic finding in EBV infection, occurring in up to 90% of patients with infectious mononucleosis who receive aminopenicillins.
*ELISA for HIV*
- While the patient has risk factors for HIV (multiple sexual partners, inconsistent condom use, prior gonorrhea), **acute HIV infection** typically presents with a more transient rash and less prominent tonsillar exudates or splenomegaly.
- An HIV ELISA is a reasonable test given his risk factors, but the overall clinical picture points more strongly to mononucleosis, and the rapid onset of rash after amoxicillin is a classic sign of EBV-associated drug reaction.
*Flow cytometry*
- **Flow cytometry** is primarily used for the diagnosis and monitoring of hematological malignancies or immunodeficiencies, not for the initial diagnosis of infectious diseases like mononucleosis.
- It would not be the next best step for evaluating the described clinical presentation.
*Anti-CMV IgM*
- **Cytomegalovirus (CMV) infection** can cause a mononucleosis-like syndrome, but it typically presents without the prominent pharyngitis/tonsillar exudates common in EBV.
- While CMV testing might be considered if the heterophile agglutination test is negative, it is not the initial best step given the classic presentation.
*Throat swab culture*
- A **throat swab culture** is primarily used to diagnose bacterial pharyngitis, such as Group A Streptococcus.
- Given the **diffuse rash after amoxicillin** and the systemic symptoms like splenomegaly and generalized lymphadenopathy, a bacterial infection is less likely to be the primary diagnosis.
- The amoxicillin-induced rash in the setting of EBV is much more diagnostically significant than streptococcal pharyngitis with a drug reaction.
Rickettsia species US Medical PG Question 6: A 2-year-old boy presents to the pediatrician for evaluation of an elevated temperature, sore throat, runny nose, and lacrimation for the past week, and a rash which he developed yesterday. The rash began on the patient’s face and spread down to the trunk, hands, and feet. The patient’s mother gave him ibuprofen to control the fever. The child has not received mumps, measles, and rubella vaccinations because he was ill when the vaccine was scheduled and was later lost to follow-up. The vital signs include blood pressure 90/50 mm Hg, heart rate 110/min, respiratory rate 22/min, and temperature 37.8°C (100.0℉). On physical examination, the child was drowsy. His face, trunk, and extremities were covered with a maculopapular erythematous rash. Two irregularly-shaped red dots were also noted on the mucosa of the lower lip. The remainder of the physical examination was within normal limits. What is the probable causative agent for this child’s condition?
- A. Group A Streptococcus
- B. Rubulavirus
- C. Influenzavirus
- D. Morbillivirus (Correct Answer)
- E. Herpesvirus
Rickettsia species Explanation: ***Morbillivirus***
- The constellation of symptoms—**elevated temperature**, **sore throat**, **runny nose**, **lacrimation**, a **maculopapular erythematous rash** that began on the face and spread downward, and especially the **irregularly-shaped red dots on the mucosa of the lower lip** (likely **Koplik spots**)—are classic for **measles**, caused by Morbillivirus.
- The patient's **unvaccinated status** against MMR further supports measles as the most probable diagnosis, as it is a highly contagious disease prevented by vaccination.
*Group A Streptococcus*
- This bacterium causes **scarlet fever**, characterized by a **sandpaper-like rash** and **strawberry tongue**, not a maculopapular rash spreading from face to extremities with Koplik spots.
- While it can cause pharyngitis and fever, the specific rash progression and oral lesions rule out Group A Streptococcus.
*Rubulavirus*
- Rubulavirus causes **mumps**, which primarily presents with **parotitis** (swelling of parotid glands), fever, and headache.
- It does not typically cause a generalized maculopapular rash or Koplik spots, making it an unlikely cause for the described symptoms.
*Influenzavirus*
- Influenzavirus causes **influenza**, characterized by sudden onset of high fever, cough, myalgia, and headache.
- While it can cause fever and respiratory symptoms, it does not typically present with a widespread maculopapular rash or Koplik spots.
*Herpesvirus*
- Herpesviruses cause a variety of conditions, including **chickenpox** (Varicella-zoster virus), which presents with **vesicular lesions** that crust over, and **roseola infantum** (HHV-6/7), which primarily causes a high fever followed by a non-pruritic rash appearing *after* the fever subsides.
- Neither of these typically presents with Koplik spots or the specific maculopapular rash progression described.
Rickettsia species US Medical PG Question 7: A 24-year-old female comes to the physician because of flu-like symptoms and a new rash for 2 days. She denies contacts with sick individuals or recent travel abroad, but recently went camping in Vermont. Vital signs are within normal limits. Examination of the lateral right thigh shows a circular red ring with central clearing. Which of the following is the natural reservoir of the pathogen responsible for this patient's symptoms?
- A. Rat
- B. Rabbit
- C. Tick
- D. Mouse (Correct Answer)
- E. Flea
Rickettsia species Explanation: ***Mouse***
- The patient's symptoms, including **flu-like illness** and a **circular red rash with central clearing** (erythema migrans) after camping in Vermont, are classic for **Lyme disease**.
- The causative agent, *Borrelia burgdorferi*, is primarily maintained in **white-footed mice** (genus *Peromyscus*) in its natural reservoir during its larval and nymphal stages.
*Rat*
- While **rats** can carry and transmit various diseases, they are not the primary natural reservoir for *Borrelia burgdorferi*, the pathogen responsible for Lyme disease.
- Diseases associated with rats often include **leptospirosis** and **plague**, which present with different clinical pictures.
*Rabbit*
- **Rabbits** are known reservoirs for diseases like **tularemia** (*Francisella tularensis*), which can cause fever, skin lesions, and lymphadenopathy, but typically not the characteristic **erythema migrans** rash.
- They are not a significant natural reservoir for *Borrelia burgdorferi*.
*Tick*
- The **tick** (specifically *Ixodes scapularis* or **deer tick**) is the **vector** that transmits *Borrelia burgdorferi* to humans, not the natural reservoir.
- The tick acquires the bacteria from infected animal hosts such as mice and deer.
*Flea*
- **Fleas** are vectors for diseases such as **bubonic plague** (*Yersinia pestis*) and **endemic typhus** (*Rickettsia typhi*), which do not manifest with erythema migrans.
- They are not involved in the transmission or natural history of **Lyme disease**.
Rickettsia species US Medical PG Question 8: 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
Rickettsia species 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.
Rickettsia species US Medical PG Question 9: A 19-year-old college student presents to student health with 1 day of fever and chills. He says that he has also been coughing for 2 days. His roommate was sick 3 days ago with similar symptoms and was diagnosed with Mycoplasma infection. He has otherwise been healthy and has had all the required vaccines as scheduled. He is currently taking introductory biology as part of his premedical studies and recently learned about antibodies. He therefore asks his physician about what his body is doing to fight off the infection. At this stage of his infection, which of the following forms are the antibodies circulating in his serum?
- A. Pentamers (Correct Answer)
- B. Dimers
- C. Tetramers
- D. Trimers
- E. Monomers
Rickettsia species Explanation: ***Pentamers***
- In the **early stages** of a primary immune response, the B cells initially produce **IgM antibodies**.
- IgM antibodies circulate in the serum primarily as **pentamers**, making them highly effective at binding multiple antigens and activating complement.
*Dimers*
- **IgA antibodies** can exist as dimers, particularly secretory IgA found in mucosal secretions, but they are not the predominant form during the initial systemic immune response to an infection.
- While IgA plays a role in immunity, IgM is the primary antibody class produced in the **first few days of a new infection**.
*Tetramers*
- Antibodies typically do not form **stable tetrameric structures** as a functional unit in serum.
- The primary forms of antibodies are monomers, dimers, and pentamers, each with specific roles and locations.
*Trimers*
- **Trimeric antibody forms** are not a standard or significant configuration for immunoglobulins circulating in the serum.
- Antibody structures are well-defined as monomers (IgG, IgE, IgD), dimers (secretory IgA), or pentamers (IgM).
*Monomers*
- While **IgM can exist as a monomer** when expressed on the surface of B cells, serum IgM is predominantly in its **pentameric form**.
- **IgG** is the most abundant monomeric antibody in serum, but it is produced later in the immune response and at higher concentrations during secondary immune responses.
Rickettsia species US Medical PG Question 10: A 21-year-old man presents to the emergency room complaining of pain upon urination and a watery discharge from his penis. It started a few days ago and has been getting progressively worse. His temperature is 98.0°F (36.7°C), blood pressure is 122/74 mmHg, pulse is 83/min, respirations are 14/min, and oxygen saturation is 98% on room air. Physical exam is notable for a tender urethra with a discharge. Gram stain of the discharge is negative for bacteria but shows many neutrophils. Which of the following is the most likely infectious etiology of this patient's symptoms?
- A. Chlamydia trachomatis (Correct Answer)
- B. Trichomonas vaginalis
- C. Neisseria gonorrhoeae
- D. Staphylococcus saprophyticus
- E. Escherichia coli
Rickettsia species Explanation: ***Chlamydia trachomatis***
- The presentation of **dysuria**, **watery discharge**, and a **Gram stain negative for bacteria but positive for neutrophils** is highly characteristic of **non-gonococcal urethritis**, with *Chlamydia trachomatis* being the most common cause.
- *Chlamydia* is an **intracellular bacterium** and does not readily stain with Gram stain, explaining the negative result despite the presence of inflammation (neutrophils).
*Trichomonas vaginalis*
- While *Trichomonas vaginalis* can cause urethritis and discharge in men, it typically presents with **frothy yellow-green discharge** and is less common than *Chlamydia* in male urethritis.
- It would also likely be identifiable on a **wet mount microscopy** rather than just a Gram stain negative for bacteria.
*Neisseria gonorrhoeae*
- **Gonococcal urethritis** typically presents with a **purulent, thick discharge** and would show **Gram-negative diplococci** on Gram stain, which are absent in this case.
- The Gram stain finding of "negative for bacteria" specifically rules out *Neisseria gonorrhoeae*.
*Staphylococcus saprophyticus*
- *Staphylococcus saprophyticus* is a common cause of **urinary tract infections (UTIs)**, especially in young women, but less commonly causes urethritis with discharge in men.
- If present, it would likely be detected on a standard **Gram stain** and culture as **Gram-positive cocci**.
*Escherichia coli*
- *Escherichia coli* is the most common cause of **UTIs** but typically causes **cystitis** or **pyelonephritis** rather than isolated urethritis with discharge in men, unless associated with specific risk factors.
- It would appear as **Gram-negative rods** on Gram stain if it were the causative agent and would typically result in a positive bacterial finding.
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