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?
Q2
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?
Q3
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?
Q4
A 23-year-old male comes to the physician because of a 2-week history of fatigue, muscle aches, and a dry cough. He has also had episodes of painful, bluish discoloration of the tips of his fingers, nose, and earlobes during this period. Three months ago, he joined the military and attended basic training in southern California. He does not smoke or use illicit drugs. His temperature is 37.8°C (100°F). Physical examination shows mildly pale conjunctivae and annular erythematous lesions with a dusky central area on the extensor surfaces of the lower extremities. Which of the following is the most likely causal organism?
Q5
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?
Q6
A previously healthy 27-year-old woman comes to the physician because of a 3-week history of fatigue, headache, and dry cough. She does not smoke or use illicit drugs. Her temperature is 37.8°C (100°F). Chest examination shows mild inspiratory crackles in both lung fields. An x-ray of the chest shows diffuse interstitial infiltrates bilaterally. A Gram stain of saline-induced sputum shows no organisms. Inoculation of the induced sputum on a cell-free medium that is enriched with yeast extract, horse serum, cholesterol, and penicillin G grows colonies that resemble fried eggs. Which of the following organisms was most likely isolated on the culture medium?
Q7
A 21-year-old college student is admitted to the emergency department with complaints of pharyngitis, headache, and a persistent, non-productive, dry, hacking cough. The patient complains of feeling tired and fatigued and denies fever/chills. On physical examination, her mucosa is pale. A complete blood count is remarkable for decreased hemoglobin. The physician suspects viral pneumonia, but the sputum culture tests come back with the following description: ‘fried-egg shaped colonies on sterol-containing media, and mulberry-shaped colonies on media containing sterols’. A direct Coombs test comes back positive. Which of the following statements is true regarding the complications associated with Mycoplasma pneumoniae?
Q8
A 34-year-old poultry worker presents to his physician with a sore throat and a non-productive cough for 2 weeks. His cough is associated with fever. The vital signs include: blood pressure 120/80 mm Hg, heart rate 67/min, respiratory rate 18/min, and temperature 37.6°C (98.0°F). Physical examination shows oropharyngeal erythema and scattered, moist rales on lung auscultation. The patient's X-ray demonstrates patchy reticular opacities in the perihilar regions of both lungs. After some additional tests, he is diagnosed with community-acquired pneumonia and is initially treated with cephalexin with no significant improvement. Which of the following best describes the immune response elicited by the pathogen that is causing this patient's condition?
Q9
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?
Q10
A 10-year-old child presents to your office with a chronic cough. His mother states that he has had a cough for the past two weeks that is non-productive along with low fevers of 100.5 F as measured by an oral thermometer. The mother denies any other medical history and states that he has been around one other friend who also has had this cough for many weeks. The patient's vitals are within normal limits with the exception of his temperature of 100.7 F. His chest radiograph demonstrated diffuse interstitial infiltrates. Which organism is most likely causing his pneumonia?
Atypical US Medical PG Practice Questions and MCQs
Question 1: 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.
Question 2: 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
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.
Question 3: 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
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.
Question 4: A 23-year-old male comes to the physician because of a 2-week history of fatigue, muscle aches, and a dry cough. He has also had episodes of painful, bluish discoloration of the tips of his fingers, nose, and earlobes during this period. Three months ago, he joined the military and attended basic training in southern California. He does not smoke or use illicit drugs. His temperature is 37.8°C (100°F). Physical examination shows mildly pale conjunctivae and annular erythematous lesions with a dusky central area on the extensor surfaces of the lower extremities. Which of the following is the most likely causal organism?
A. Chlamydophila pneumoniae
B. Streptococcus pneumoniae
C. Mycoplasma pneumoniae (Correct Answer)
D. Adenovirus
E. Influenza virus
Explanation: ***Mycoplasma pneumoniae***
- This patient's symptoms (fatigue, muscle aches, dry cough, slightly elevated temperature, and **erythema multiforme**-like lesions) are characteristic of **atypical pneumonia**. The **Raynaud-like phenomenon** (bluish discoloration of fingertips, nose, earlobes) and recent military basic training environment are highly suggestive of *Mycoplasma pneumoniae* infection.
- *Mycoplasma pneumoniae* is a common cause of **atypical pneumonia**, especially in crowded settings like military barracks, and is associated with extrapulmonary manifestations such as **Raynaud's phenomenon**, **hemolytic anemia** (suggested by pale conjunctivae), and **erythema multiforme**.
*Chlamydophila pneumoniae*
- This organism also causes **atypical pneumonia** with a dry cough and constitutional symptoms but is less commonly associated with the prominent extrapulmonary findings like **Raynaud's phenomenon** and **erythema multiforme** seen in this patient.
- While it can cause pharyngitis and hoarseness, the constellation of symptoms, particularly the cutaneous and vascular manifestations, points away from *Chlamydophila pneumoniae*.
*Streptococcus pneumoniae*
- *Streptococcus pneumoniae* typically causes **typical bacterial pneumonia**, characterized by a **productive cough**, high fever, chills, and often **lobar consolidation** on chest X-ray.
- It is not associated with **Raynaud's phenomenon**, **erythema multiforme**, or the specific demographic and exposure history (military basic training for atypical presentation) described.
*Adenovirus*
- **Adenovirus** can cause **respiratory tract infections**, including pharyngitis, conjunctivitis, and pneumonia, often seen in outbreaks in crowded settings.
- However, it is less commonly associated with the dramatic extrapulmonary manifestations like **Raynaud's phenomenon** and **erythema multiforme** that are prominent in this case.
*Influenza virus*
- **Influenza virus** causes a **respiratory illness** with fever, myalgia, fatigue, and cough, but **dry cough** is more common.
- While it can lead to pneumonia, the presence of **Raynaud's phenomenon** and **erythema multiforme** is not a typical presentation of influenza.
Question 5: 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 6: A previously healthy 27-year-old woman comes to the physician because of a 3-week history of fatigue, headache, and dry cough. She does not smoke or use illicit drugs. Her temperature is 37.8°C (100°F). Chest examination shows mild inspiratory crackles in both lung fields. An x-ray of the chest shows diffuse interstitial infiltrates bilaterally. A Gram stain of saline-induced sputum shows no organisms. Inoculation of the induced sputum on a cell-free medium that is enriched with yeast extract, horse serum, cholesterol, and penicillin G grows colonies that resemble fried eggs. Which of the following organisms was most likely isolated on the culture medium?
A. Bordetella pertussis
B. Haemophilus influenzae
C. Cryptococcus neoformans
D. Mycoplasma pneumoniae (Correct Answer)
E. Coxiella burnetii
Explanation: ***Mycoplasma pneumoniae***
- The patient's presentation with **atypical pneumonia** symptoms (fatigue, headache, dry cough, diffuse interstitial infiltrates) along with the characteristic **"fried egg" colonies** cultured on a specialized medium (enriched with yeast extract, horse serum, cholesterol, and penicillin G) are highly indicative of *Mycoplasma pneumoniae*.
- Unlike most bacteria, *Mycoplasma pneumoniae* lacks a **cell wall**, explaining why it does not stain on Gram stain and requires specific culture conditions.
*Bordetella pertussis*
- This organism causes **whooping cough** and is characterized by distinct paroxysmal cough followed by an inspiratory 'whoop,' not typically the diffuse interstitial infiltrates and mild crackles described.
- *Bordetella pertussis* is usually cultured on **Bordet-Gengou agar** or Regan-Lowe medium, not the specialized medium described, and does not form "fried egg" colonies.
*Haemophilus influenzae*
- This bacterium is a common cause of **bacterial pneumonia** but typically presents with more acute symptoms and localized infiltrates, often seen in individuals with underlying lung disease or in children.
- *Haemophilus influenzae* would grow on standard chocolate agar and would not produce "fried egg" colonies; it also stains Gram-negative coccobacilli.
*Cryptococcus neoformans*
- This is a **fungus**, not a bacterium, and is a significant cause of pneumonia and meningoencephalitis, especially in immunocompromised individuals.
- Its presence would typically be identified by **India ink stain** (showing encapsulated yeast) or specific fungal cultures, not the described "fried egg" appearance on bacterial media.
*Coxiella burnetii*
- This intracellular bacterium causes **Q fever**, which can present with atypical pneumonia symptoms, but it is an **obligate intracellular parasite** and therefore cannot be grown on cell-free artificial media like the one described.
- Diagnosis typically relies on **serological tests** or PCR, as it cannot be easily cultured in a standard lab setup.
Question 7: A 21-year-old college student is admitted to the emergency department with complaints of pharyngitis, headache, and a persistent, non-productive, dry, hacking cough. The patient complains of feeling tired and fatigued and denies fever/chills. On physical examination, her mucosa is pale. A complete blood count is remarkable for decreased hemoglobin. The physician suspects viral pneumonia, but the sputum culture tests come back with the following description: ‘fried-egg shaped colonies on sterol-containing media, and mulberry-shaped colonies on media containing sterols’. A direct Coombs test comes back positive. Which of the following statements is true regarding the complications associated with Mycoplasma pneumoniae?
A. It is similarly associated with systemic lupus erythematosus
B. The underlying mechanism is complement-independent.
C. Red blood cells bind to IgG in warm temperatures > 37°C (98.6°F)
D. Red blood cells bind to IgM in cold temperatures < 37°C (98.6°F) (Correct Answer)
E. It primarily causes intravascular hemolysis rather than extravascular hemolysis
Explanation: ***Red blood cells bind to IgM in cold temperatures < 37°C (98.6°F)***
- This patient's presentation, including **non-productive cough**, **fatigue**, and **positive Coombs test**, along with the characteristic **"fried-egg"** and **"mulberry" colonies** (consistent with *Mycoplasma pneumoniae*), indicates a secondary **cold agglutinin hemolytic anemia**.
- In response to *Mycoplasma pneumoniae* infection, the body produces **IgM antibodies** that react with **I antigen** on red blood cells, leading to agglutination and hemolysis preferentially at temperatures below **37°C (98.6°F)** in cooler areas of the body.
*It is similarly associated with systemic lupus erythematosus*
- **Systemic lupus erythematosus** (SLE) is an autoimmune disease primarily associated with **warm autoimmune hemolytic anemia (AIHA)**, where IgG antibodies are active at body temperature.
- While both can cause anemia, the immunological mechanism and primary antibodies involved are distinct.
*The underlying mechanism is complement-independent.*
- **Cold agglutinin disease** is a **complement-dependent** process; IgM antibodies cause agglutination, which then activates the classical complement pathway leading to C3b deposition on red blood cells.
- The opsonized red blood cells are subsequently cleared by macrophages in the liver and spleen (extravascular hemolysis) or lysed intravascularly by the full complement cascade.
*Red blood cells bind to IgG in warm temperatures > 37°C (98.6°F)*
- This describes **warm autoimmune hemolytic anemia (AIHA)**, where **IgG antibodies** bind to red blood cells optimally at **body temperature (37°C)**.
- The patient's condition, with a positive direct Coombs test but features suggestive of *Mycoplasma pneumoniae* and positive complement, is indicative of **cold agglutinin disease**, which is mediated by IgM.
*It primarily causes intravascular hemolysis rather than extravascular hemolysis*
- Although some **intravascular hemolysis** can occur, **cold agglutinin disease** caused by *Mycoplasma pneumoniae* primarily results in **extravascular hemolysis**, particularly in the liver and spleen.
- Macrophages in these organs recognize and clear red blood cells opsonized with C3b, which is deposited after IgM binds and activates complement, leading to red blood cell destruction outside the bloodstream.
Question 8: A 34-year-old poultry worker presents to his physician with a sore throat and a non-productive cough for 2 weeks. His cough is associated with fever. The vital signs include: blood pressure 120/80 mm Hg, heart rate 67/min, respiratory rate 18/min, and temperature 37.6°C (98.0°F). Physical examination shows oropharyngeal erythema and scattered, moist rales on lung auscultation. The patient's X-ray demonstrates patchy reticular opacities in the perihilar regions of both lungs. After some additional tests, he is diagnosed with community-acquired pneumonia and is initially treated with cephalexin with no significant improvement. Which of the following best describes the immune response elicited by the pathogen that is causing this patient's condition?
A. Antibody-mediated immunity plays the leading role in the elimination of this pathogen.
B. It primarily induces the Th1-cell response. (Correct Answer)
C. Peptidoglycan is its major antigen that induces an immune response.
D. This pathogen evades the immune response by encapsulation.
E. It activates TLR5 on the surface of macrophages.
Explanation: ***It primarily induces the Th1-cell response.***
- The patient's symptoms (poultry worker, sore throat, non-productive cough, fever, patchy reticular opacities, modest improvement with cephalexin) are highly suggestive of **Chlamydophila psittaci** pneumonia (**psittacosis**).
- As an **obligate intracellular bacterium**, Chlamydophila psittaci primarily elicits a **cell-mediated immune response**, specifically a **Th1-cell response**, which is crucial for clearing intracellular pathogens.
*Antibody-mediated immunity plays the leading role in the elimination of this pathogen.*
- While antibodies play a role, **humoral immunity** is generally less effective against **intracellular pathogens** like Chlamydophila psittaci because antibodies cannot access the pathogen within host cells.
- The primary defense against intracellular bacteria relies on **cell-mediated immunity** to destroy infected cells or activate macrophages.
*Peptidoglycan is its major antigen that induces an immune response.*
- **Chlamydophila psittaci** lacks a conventional **peptidoglycan layer** in its cell wall, distinguishing it from most other bacteria.
- Therefore, **peptidoglycan** is not a major antigen that induces an immune response against this pathogen.
*This pathogen evades the immune response by encapsulation.*
- **Chlamydia** species, including Chlamydophila psittaci, do **not possess a capsule** as a primary mechanism of immune evasion.
- Their main evasion strategy is their **intracellular lifestyle**, which protects them from humoral immune responses.
*It activates TLR5 on the surface of macrophages.*
- **TLR5** specifically recognizes **flagellin**, a protein component of bacterial flagella.
- **Chlamydophila psittaci** is a **non-motile bacterium** and does not possess flagella, so it would not activate TLR5.
Question 9: 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
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*.
Question 10: A 10-year-old child presents to your office with a chronic cough. His mother states that he has had a cough for the past two weeks that is non-productive along with low fevers of 100.5 F as measured by an oral thermometer. The mother denies any other medical history and states that he has been around one other friend who also has had this cough for many weeks. The patient's vitals are within normal limits with the exception of his temperature of 100.7 F. His chest radiograph demonstrated diffuse interstitial infiltrates. Which organism is most likely causing his pneumonia?
A. Mycoplasma pneumoniae (Correct Answer)
B. Streptococcus agalactiae
C. Staphylococcus aureus
D. Pneumocystis jiroveci
E. Streptococcus pneumoniae
Explanation: ***Mycoplasma pneumoniae***
- This presentation of a **subacute onset** of a **non-productive cough** with **low-grade fevers** and **diffuse interstitial infiltrates** on chest X-ray in a school-aged child is highly characteristic of **atypical pneumonia** caused by *Mycoplasma pneumoniae*.
- The history of exposure to an infected friend further supports community spread of this organism, which commonly causes outbreaks of "walking pneumonia."
*Streptococcus agalactiae*
- This organism primarily causes infections in **neonates** (Group B Strep) and pregnant women, such as **sepsis** and **meningitis**, and is not a common cause of pneumonia in a 10-year-old.
- Pulmonary infections in older children due to *S. agalactiae* are rare and typically occur in those with significant comorbidities.
*Staphylococcus aureus*
- Pneumonia due to *Staphylococcus aureus* often presents with a more **acute and severe course**, including high fevers, productive cough, and sometimes **abscess formation** or **empyema** on imaging.
- It's commonly associated with preceding influenza infection, intravenous drug use, or hospitalization, none of which are described.
*Pneumocystis jiroveci*
- *Pneumocystis jiroveci* pneumonia (PJP) is almost exclusively seen in **immunocompromised individuals**, such as those with HIV/AIDS, organ transplant recipients, or those on immunosuppressive medications.
- The patient described is a healthy 10-year-old child with no history of immunosuppression.
*Streptococcus pneumoniae*
- **Pneumococcal pneumonia** typically presents with a **sudden onset** of high fever, **productive cough** with rusty sputum, and a more localized lobar infiltrate on chest X-ray.
- While common in children, the subacute, non-productive nature of the cough and the diffuse interstitial infiltrates are not typical for *S. pneumoniae*.
