Microbiome functions and host defense US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Microbiome functions and host defense. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Microbiome functions and host defense US Medical PG Question 1: A medical student is reading about a specific type of T cells that plays an important role in immunologic tolerance. Most of these cells develop in the thymus, but some of them also develop in peripheral lymphoid organs. Usually, they are CD4+ cells and also express CD25 molecules. The functions of these cells are dependent on forkhead box P3 (Foxp3). Their function is to block the activation of lymphocytes that could react with self-antigens in a potentially harmful manner. Which of the following interleukins is secreted by these cells?
- A. Interleukin-6
- B. Interleukin-10 (Correct Answer)
- C. Interleukin-2
- D. Interleukin-12
- E. Interleukin-17
Microbiome functions and host defense Explanation: ***Interleukin-10***
- The description points to **regulatory T cells (Tregs)**, which are CD4+, CD25+, and Foxp3+. A key function of Tregs in maintaining **immunologic tolerance** is the secretion of **IL-10** and TGF-β.
- **IL-10** is a potent **anti-inflammatory cytokine** that suppresses the activation and proliferation of various immune cells, including T cells, macrophages, and dendritic cells, thereby preventing immune responses against self-antigens.
*Interleukin-6*
- **IL-6** is a **pro-inflammatory cytokine** primarily involved in the acute phase response, hematopoiesis, and differentiation of Th17 cells, which is contrary to the immunosuppressive role of Tregs.
- It promotes inflammation and is secreted by various cells, including macrophages, T cells, and B cells, but not typically by Tregs as part of their suppressive function.
*Interleukin-2*
- **IL-2** is an important **T cell growth factor**, crucial for the proliferation and differentiation of T cells, including Tregs themselves, but it is primarily secreted by activated helper T cells (Th1).
- While Tregs express the **CD25 (IL-2 receptor alpha chain)** and require IL-2 for their survival and function, they do not typically secrete IL-2 as their primary immunomodulatory cytokine.
*Interleukin-12*
- **IL-12** is a cytokine mainly produced by antigen-presenting cells (APCs) like dendritic cells and macrophages, and plays a critical role in promoting **Th1 differentiation** and cell-mediated immunity.
- It is a **pro-inflammatory cytokine** that drives immune responses, which is opposite to the suppressive function described for these cells.
*Interleukin-17*
- **IL-17** is the signature cytokine of **Th17 cells**, which are primarily involved in host defense against extracellular bacteria and fungi, but also play a significant role in mediating autoimmune diseases.
- It is a **pro-inflammatory cytokine** and its production is antagonistic to the immunosuppressive function of regulatory T cells.
Microbiome functions and host defense US Medical PG Question 2: A 37-year-old woman with a history of anorectal abscesses complains of pain in the perianal region. Physical examination reveals mild swelling, tenderness, and erythema of the perianal skin. She is prescribed oral ampicillin and asked to return for follow-up. Two days later, the patient presents with a high-grade fever, syncope, and increased swelling. Which of the following would be the most common mechanism of resistance leading to the failure of antibiotic therapy in this patient?
- A. Intrinsic absence of a target site for the drug
- B. Use of an altered metabolic pathway
- C. Production of beta-lactamase enzyme (Correct Answer)
- D. Altered structural target for the drug
- E. Drug efflux pump
Microbiome functions and host defense Explanation: ***Production of beta-lactamase enzyme***
- The patient's symptoms of a rapidly worsening infection despite ampicillin treatment suggest the presence of a **beta-lactamase producing organism**. Ampicillin is a **beta-lactam antibiotic** that is inactivated by these enzymes.
- Anorectal abscesses and rapidly progressing soft tissue infections are often caused by **polymicrobial flora**, including staphylococci and enterococci, many of which can produce **beta-lactamase**.
*Intrinsic absence of a target site for the drug*
- While some bacteria inherently lack the target site for certain drugs (e.g., mycoplasma lacking a cell wall, thus being resistant to beta-lactams), this is less likely to be the **most common mechanism of acquired resistance** leading to treatment failure in a typical perianal infection.
- The rapid progression and failed initial treatment point towards an **acquired mechanism of resistance** rather than an intrinsic one.
*Use of an altered metabolic pathway*
- This mechanism, such as altered **folate synthesis pathways** in resistance to trimethoprim-sulfamethoxazole, is less common as the primary mechanism for ampicillin resistance.
- Ampicillin's mechanism of action primarily targets the **bacterial cell wall**, not a metabolic pathway in the same way.
*Altered structural target for the drug*
- This involves modifications to the **penicillin-binding proteins (PBPs)**, which are the targets of beta-lactam antibiotics like ampicillin. While a valid mechanism (e.g., in MRSA), the **production of beta-lactamase** is generally a more widespread and common cause of ampicillin failure, especially in infections involving mixed flora from the perianal region.
- Given the abrupt failure of ampicillin, **beta-lactamase inactivation** is a more immediate and common cause than a rapid mutational change in PBPs.
*Drug efflux pump*
- **Efflux pumps** actively remove antibiotics from the bacterial cell, contributing to resistance against various drug classes.
- While efflux pumps can play a role, the dominant mechanism for resistance to **ampicillin** in many common perianal pathogens is the **enzymatic degradation by beta-lactamases**.
Microbiome functions and host defense US Medical PG Question 3: An investigator is studying the effect of different cytokines on the growth and differentiation of B cells. The investigator isolates a population of B cells from the germinal center of a lymph node. After exposure to a particular cytokine, these B cells begin to produce an antibody that prevents attachment of pathogens to mucous membranes but does not fix complement. Which of the following cytokines is most likely responsible for the observed changes in B-cell function?
- A. Interleukin-5 (Correct Answer)
- B. Interleukin-6
- C. Interleukin-8
- D. Interleukin-2
- E. Interleukin-4
Microbiome functions and host defense Explanation: ***Interleukin-5***
- The antibody described (prevents pathogen attachment to mucous membranes, does not fix complement) is characteristic of **IgA**.
- **IL-5** plays a crucial role in promoting **IgA secretion** by differentiated B cells and supports B cell growth and differentiation in mucosal immunity.
- IL-5 works synergistically with **TGF-β** (the primary cytokine for IgA class switching) to enhance IgA production, particularly in mucosal-associated lymphoid tissue.
- Among the options provided, **IL-5 has the strongest association with IgA production**.
*Interleukin-4*
- **IL-4** is the primary cytokine driving class switching to **IgE** (and IgG4), not IgA.
- IL-4 is central to **allergic responses** and type 2 immunity, promoting B cells to produce IgE antibodies against allergens and parasites.
- It does not play a significant role in IgA production or mucosal immunity.
*Interleukin-6*
- **IL-6** is a pleiotropic cytokine involved in acute phase reactions, inflammation, and promoting B cell **differentiation into plasma cells**.
- While it supports general B cell maturation and antibody secretion, it is not specifically associated with **IgA production** or class switching.
*Interleukin-8*
- **IL-8** (CXCL8) is a **chemokine** that primarily recruits and activates neutrophils during inflammation.
- It has no direct role in B cell class switching or antibody production.
*Interleukin-2*
- **IL-2** is essential for T cell proliferation and differentiation, enhancing **cell-mediated immunity**.
- While it can indirectly affect B cell responses through T cell help, it is not directly responsible for promoting **IgA production** or class switching.
Microbiome functions and host defense US Medical PG Question 4: A 72-year-old patient presents to the emergency department because of abdominal pain, diarrhea, and fever. He was started on levofloxacin for community-acquired pneumonia 2 weeks prior with resolution of his pulmonary symptoms. He has had hypertension for 20 years, for which he takes amlodipine. His temperature is 38.3°C (101.0°F), pulse is 90/min, and blood pressure is 110/70 mm Hg. On examination, mild abdominal distension with minimal tenderness was found. Laboratory tests reveal a peripheral white blood cell count of 12.000/mm3 and a stool guaiac mildly positive for occult blood. Which of the following best describe the mechanism of this patient illness?
- A. Damage to the gastrointestinal tract by enteropathogenic viruses
- B. Autoimmune inflammation of the rectum
- C. Disruption of normal bowel flora and infection by spore-forming rods (Correct Answer)
- D. Decreased blood flow to the gastrointestinal tract
- E. Presence of osmotically active, poorly absorbed solutes in the bowel lumen
Microbiome functions and host defense Explanation: ***Disruption of normal bowel flora and infection by spore-forming rods***
- This describes **Clostridioides difficile infection (CDI)**, which is strongly suggested by the patient's recent antibiotic use (levofloxacin, a fluoroquinolone) followed by abdominal pain, diarrhea, fever, and leukocytosis.
- Antibiotics disrupt the normal gut microbiome, allowing **C. difficile (spore-forming rods)** to proliferate and produce toxins that cause colitis.
*Damage to the gastrointestinal tract by enteropathogenic viruses*
- While viral gastroenteritis can cause these symptoms, the **recent history of antibiotic use** makes CDI a much more likely diagnosis.
- Viral infections typically resolve spontaneously and are less likely to cause a significant **leukocytosis** and **occult blood in stool** in this context.
*Autoimmune inflammation of the rectum*
- Conditions like **ulcerative colitis**, an autoimmune disease, can cause similar symptoms but typically have a **chronic or relapsing course** and are not usually triggered by recent antibiotic use.
- The acute presentation following antibiotics strongly points away from an autoimmune process.
*Decreased blood flow to the gastrointestinal tract*
- **Ischemic colitis** can cause abdominal pain and bloody diarrhea, especially in older patients with vascular risk factors (like hypertension).
- However, the prominent **fever** and **leukocytosis**, coupled with recent antibiotic use, are more indicative of an infectious process like CDI than ischemia.
*Presence of osmotically active, poorly absorbed solutes in the bowel lumen*
- This mechanism describes **osmotic diarrhea**, which can be caused by malabsorption (e.g., lactose intolerance) or certain laxatives.
- Osmotic diarrhea typically **resolves with fasting** and is not usually associated with fever, significant leukocytosis, or occult blood in the stool, which are present here.
Microbiome functions and host defense US Medical PG Question 5: A 57-year-old HIV-positive male with a history of intravenous drug abuse presents to the emergency room complaining of arm swelling. He reports that he developed progressively worsening swelling and tenderness over the right antecubital fossa three days prior. He recently returned from a trip to Nicaragua. His past medical history is notable for an anaphylactoid reaction to vancomycin. His temperature is 101.4°F (38.6°C), blood pressure is 140/70 mmHg, pulse is 110/min, and respirations are 20/min. Physical examination reveals an erythematous, fluctuant, and tender mass overlying the right antecubital fossa. Multiple injection marks are noted across both upper extremities. He undergoes incision and drainage and is started on an antibiotic that targets the 50S ribosome. He is discharged with plans to follow up in one week. However, five days later he presents to the same emergency room complaining of abdominal cramps and watery diarrhea. Which of the following classes of pathogens is most likely responsible for this patient’s current symptoms?
- A. Gram-negative curved bacillus
- B. Gram-negative bacillus
- C. Anaerobic flagellated protozoan
- D. Gram-positive bacillus (Correct Answer)
- E. Gram-positive coccus
Microbiome functions and host defense Explanation: ***Gram-positive bacillus***
- The patient was administered an antibiotic targeting the **50S ribosomal subunit** following incision and drainage for a suspected skin infection (likely **MRSA** given IV drug abuse). This strongly suggests **clindamycin** was used.
- **Clindamycin** is a known risk factor for developing **Clostridioides (formerly Clostridium) difficile infection (CDI)**, which is caused by a **Gram-positive, spore-forming bacillus** and manifests with **abdominal cramps and watery diarrhea**.
*Gram-negative curved bacillus*
- This class of pathogens includes organisms like **Vibrio cholerae** or **Campylobacter jejuni**, which can cause diarrhea.
- However, the patient's presentation with **colitis** after antibiotic use is more consistent with **Clostridioides difficile**, not typically a curved Gram-negative bacillus.
*Gram-negative bacillus*
- While some Gram-negative bacilli (e.g., E. coli, Salmonella) can cause diarrhea, their association with **antibiotic-induced colitis** following treatment for a skin abscess is less direct than that of *Clostridioides difficile*.
- The initial skin infection in IV drug users is most commonly staphylococcal (Gram-positive coccus), for which a 50S targeting antibiotic would be prescribed.
*Anaerobic flagellated protozoan*
- This description often refers to pathogens like **Giardia lamblia** or **Trichomonas vaginalis**, which are not bacteria.
- While *Giardia* can cause diarrhea, it typically causes **malabsorption** and **greasy stools**, and wouldn't be triggered by recent antibiotic use for a skin infection.
*Gram-positive coccus*
- **Gram-positive cocci** (e.g., Staphylococcus aureus) are the likely cause of the initial skin infection/abscess.
- However, they do not typically cause **antibiotic-associated colitis** with watery diarrhea; rather, the *antibiotic treatment itself* for these organisms can predispose to *Clostridioides difficile*.
Microbiome functions and host defense US Medical PG Question 6: A 17-year-old girl is brought in by her mother due to rapid weight loss over the past month. The patient says she has been having episodes of diarrhea, which she attributes to laxatives she takes regularly to keep her weight down. She also says she has not had her period yet. The patient’s mother adds that the patient has been underperforming at school and acting very strangely at home. Her current BMI is 16.8 kg/m2. On physical examination, the skin on her limbs and around her neck is inflamed and erythematous. Her tongue is bright red and smooth. She states that over the last 2 weeks, she has been eating nothing but small portions of fruit. She is diagnosed with a vitamin deficiency. Which of the following statements is true about the vitamin most likely deficient in this patient?
- A. It increases the GI absorption of iron
- B. It is derived from tyrosine
- C. Synthesis requires vitamin B2 and B6 (Correct Answer)
- D. Synthesis requires vitamin B1 and B6
- E. It is used to treat hypertension
Microbiome functions and host defense Explanation: ***Synthesis requires vitamin B2 and B6***
- The patient's symptoms (diarrhea, dermatitis, dementia-like behavior, glossitis, and weight loss) are classic for **pellagra**, which is caused by a deficiency in **niacin (vitamin B3)**.
- The synthesis of **niacin** from **tryptophan** requires **pyridoxine (vitamin B6)** and **riboflavin (vitamin B2)** as cofactors.
*It increases the GI absorption of iron*
- **Vitamin C** (ascorbic acid) enhances the **gastrointestinal absorption of non-heme iron** by reducing ferric iron to its ferrous form.
- Niacin does not play a direct role in the absorption of iron.
*It is derived from tyrosine*
- **Tyrosine** is a precursor to several important compounds, including **catecholamines** (dopamine, norepinephrine, epinephrine) and **thyroid hormones**.
- **Niacin** is predominantly synthesized from the essential amino acid **tryptophan**.
*Synthesis requires vitamin B1 and B6*
- While **vitamin B6** is essential for niacin synthesis from tryptophan, **vitamin B1 (thiamine)** is not directly involved in this pathway.
- Thiamine's primary role is in carbohydrate metabolism.
*It is used to treat hypertension*
- While **niacin** can affect lipid profiles, it is **not commonly used as a primary treatment for hypertension**.
- **Niacin** is used, primarily in pharmacologic doses, to **lower LDL cholesterol** and **triglycerides** and **raise HDL cholesterol**, often in conjunction with other lipid-lowering agents.
Microbiome functions and host defense US Medical PG Question 7: A 21-year-old woman comes to the physician because of a 4-day history of abdominal cramps and bloody diarrhea 5 times per day. Her symptoms began after she ate an egg sandwich from a restaurant. Her vital signs are within normal limits. Physical examination shows diffuse abdominal tenderness. Stool culture shows gram-negative rods that produce hydrogen sulfide and do not ferment lactose. Which of the following effects is most likely to occur if she receives antibiotic therapy?
- A. Orange discoloration of bodily fluids
- B. Pruritic maculopapular rash on the extensor surface
- C. Self-limiting systemic inflammatory response
- D. Prolonged fecal excretion of the pathogen (Correct Answer)
- E. Thrombocytopenia and hemolytic anemia
Microbiome functions and host defense Explanation: ***Prolonged fecal excretion of the pathogen***
- The patient's symptoms (abdominal cramps, bloody diarrhea after eating an egg sandwich) and stool culture results (gram-negative rods, hydrogen sulfide producers, non-lactose fermenting) are highly suggestive of **Salmonella enterica** infection.
- Antibiotic treatment for non-typhoidal Salmonella gastroenteritis typically **prolongs fecal excretion** and does not shorten the illness, reserving antibiotics for severe cases or immunocompromised individuals.
*Orange discoloration of bodily fluids*
- **Orange discoloration of bodily fluids** (urine, sweat, tears) is a known side effect of **rifampin**, an antibiotic primarily used for tuberculosis and some bacterial meningitides.
- Rifampin is not indicated nor commonly used for Salmonella gastroenteritis.
*Pruritic maculopapular rash on the extensor surface*
- A **pruritic maculopapular rash on the extensor surfaces** is a common presentation of drug reactions, often associated with **penicillins** or **cephalosporins**, especially in viral infections (e.g., amoxicillin rash in mononucleosis).
- This is a general antibiotic side effect and not specifically linked to the outcome of treating Salmonella.
*Self-limiting systemic inflammatory response*
- A self-limiting systemic inflammatory response could be a general reaction to an active infection or a drug, but it's not the most likely or specific outcome of **antibiotic therapy in Salmonella gastroenteritis**.
- Worsening of symptoms can occur in some cases due to toxemia from bacterial lysis (e.g., Jarisch-Herxheimer reaction), but "self-limiting systemic inflammatory response" is too generic for this specific scenario.
*Thrombocytopenia and hemolytic anemia*
- **Thrombocytopenia and hemolytic anemia** in the setting of diarrheal illness strongly suggest **hemolytic uremic syndrome (HUS)**, which is typically associated with **Shiga toxin-producing E. coli** (STEC), particularly E. coli O157:H7.
- While Salmonella can cause severe disease, HUS is not a typical complication of its treatment, and antibiotics are often avoided in STEC infections due to increased risk of HUS.
Microbiome functions and host defense US Medical PG Question 8: A 34-year-old woman presents with confusion, drowsiness, and headache. The patient’s husband says her symptoms began 2 days ago and have progressively worsened with an acute deterioration of her mental status 2 hours ago. The patient describes the headaches as severe, localized to the frontal and periorbital regions, and worse in the morning. Review of symptoms is significant for a mild, low-grade fever, fatigue, and nausea for the past week. Past medical history is significant for HIV infection for which she is not currently receiving therapy. Her CD4+ T cell count last month was 250/mm3. The blood pressure is 140/85 mm Hg, the pulse rate is 90/min, and the temperature is 37.7°C (100.0°F). On physical examination, the patient is conscious but drowsy. Papilledema is present. No pain is elicited with extension of the leg at the knee joint. The remainder of the physical examination is negative. Laboratory findings, including panculture, are ordered. A noncontrast CT scan of the head is negative and is followed by a lumbar puncture. CSF analysis is significant for:
Opening pressure 250 mm H2O (70-180 mm H2O)
Glucose 30 mg/dL (40-70 mg/dL)
Protein 100 mg/dL (<40 mg/dL)
Cell count 20/mm3 (0-5/mm3)
Which of the following additional findings would most likely be found in this patient?
- A. Gram-positive diplococci are present on microscopy
- B. CSF shows a positive acid-fast bacillus stain
- C. Multiple ring-enhancing lesions are seen on a CT scan
- D. CSF shows gram negative diplococci
- E. CSF India ink stain shows encapsulated yeast cells (Correct Answer)
Microbiome functions and host defense Explanation: ***CSF India ink stain shows encapsulated yeast cells***
- The patient's presentation with **subacute meningitis symptoms** (headache, confusion, low-grade fever) in the setting of **untreated HIV infection** with a low CD4+ count (250/mm3) strongly suggests an opportunistic infection.
- The CSF findings of **elevated opening pressure**, **low glucose**, **high protein**, and **moderate pleocytosis** are classic for **cryptococcal meningitis**, for which the India ink stain is diagnostic for encapsulated yeast cells.
*Gram-positive diplococci are present on microscopy*
- This finding suggests **bacterial meningitis**, specifically caused by organisms like *Streptococcus pneumoniae*.
- While bacterial meningitis presents acutely with severe symptoms, the **subacute course** and moderate pleocytosis are less typical, and the patient's immune status points towards an opportunistic infection.
*CSF shows a positive acid-fast bacillus stain*
- A positive **acid-fast bacillus (AFB) stain** in CSF would indicate **tuberculous meningitis**.
- While tuberculous meningitis can present subacutely with similar CSF findings in HIV patients, it typically involves a more significant lymphocytic pleocytosis and a more pronounced chronic course than suggested by the acute worsening.
*Multiple ring-enhancing lesions are seen on a CT scan*
- **Multiple ring-enhancing lesions** on CT or MRI are characteristic of **Toxoplasma encephalopathy** or **CNS lymphoma** in HIV-positive patients.
- While these are common HIV-related CNS complications, the patient's primary presentation points to **meningitis** (inflammation of meninges with CSF abnormalities) rather than focal brain lesions without meningeal involvement.
*CSF shows gram negative diplococci*
- **Gram-negative diplococci** in CSF suggest **meningococcal meningitis** (*Neisseria meningitidis*).
- This typically presents as an **acute, severe bacterial meningitis** with rapid deterioration, usually in immunocompetent individuals or specific outbreaks, which doesn't align with the subacute onset and specific CSF profile for cryptococcus.
Microbiome functions and host defense US Medical PG Question 9: A 68-year-old man comes to the physician because of a 1-month history of fatigue, low-grade fevers, and cough productive of blood-tinged sputum. He has type 2 diabetes mellitus and chronic kidney disease and underwent kidney transplantation 8 months ago. His temperature is 38.9°C (102.1°F) and pulse is 98/min. Examination shows rhonchi in the right lower lung field. An x-ray of the chest shows a right-sided lobar consolidation. A photomicrograph of specialized acid-fast stained tissue from a blood culture is shown. Which of the following is the strongest predisposing factor for this patient's condition?
- A. Sharing of unsterile IV needles
- B. Poor oral hygiene
- C. Exposure to contaminated air-conditioning unit
- D. Exposure to contaminated soil (Correct Answer)
- E. Crowded living situation
Microbiome functions and host defense Explanation: ***Exposure to contaminated soil***
- The photomicrograph shows **acid-fast stain** demonstrating **filamentous, branching gram-positive rods**, consistent with **Nocardia species**.
- **Nocardiosis** is acquired through **inhalation of Nocardia spores from contaminated soil or dust**, which is the primary environmental source and route of transmission.
- While this patient's **immunocompromised status** (post-kidney transplant on immunosuppressive therapy) is the critical host factor that predisposes him to infection, **soil exposure** is the specific environmental predisposing factor among the options listed.
- Nocardia is an opportunistic pathogen that primarily affects immunocompromised individuals, causing pulmonary infection that can disseminate.
*Sharing of unsterile IV needles*
- Sharing unsterile IV needles is a common route for transmitting **bloodborne pathogens** (e.g., HIV, hepatitis B/C) or causing bacterial endocarditis and soft tissue infections.
- This is not the typical route of acquisition for **pulmonary nocardiosis**, which is acquired via inhalation.
*Poor oral hygiene*
- Poor oral hygiene predisposes to dental caries, periodontal disease, and aspiration of oral flora causing pneumonia or lung abscess.
- **Actinomyces** (not acid-fast) is associated with poor oral hygiene and can be confused with Nocardia morphologically, but Actinomyces is not acid-fast positive.
- This is not a risk factor for acquiring **Nocardia** infection.
*Exposure to contaminated air-conditioning unit*
- Contaminated air-conditioning units and water systems are associated with **Legionella pneumophila**, causing Legionnaires' disease.
- Legionella is not acid-fast and does not show the branching filamentous morphology seen with Nocardia.
*Crowded living situation*
- Crowded living situations increase risk of person-to-person transmission of respiratory pathogens such as **Mycobacterium tuberculosis**, influenza, and other droplet-spread infections.
- **Nocardia** is acquired from environmental sources (soil, dust), not through person-to-person transmission.
Microbiome functions and host defense US Medical PG Question 10: A 35-year-old man comes to the emergency department with fever, chills, dyspnea, and a productive cough. His symptoms began suddenly 2 days ago. He was diagnosed with HIV 4 years ago and has been on triple antiretroviral therapy since then. He smokes one pack of cigarettes daily. He is 181 cm (5 ft 11 in) tall and weighs 70 kg (154 lb); BMI is 21.4 kg/m2. He lives in Illinois and works as a carpenter. His temperature is 38.8°C (101.8°F), pulse is 110/min, respirations are 24/min, and blood pressure is 105/74 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 92%. Examinations reveals crackles over the right lower lung base. The remainder of the examination shows no abnormalities. Laboratory studies show:
Hemoglobin 11.5 g/dL
Leukocyte count 12,800/mm3
Segmented neutrophils 80%
Eosinophils 1%
Lymphocytes 17%
Monocytes 2%
CD4+ T-lymphocytes 520/mm3(N ≥ 500)
Platelet count 258,000/mm3
Serum
Na+ 137 mEq/L
Cl- 102 mEq/L
K+ 5.0 mEq/L
HCO3- 22 mEq/L
Glucose 92 mg/dL
An x-ray of the chest shows a right lower-lobe infiltrate of the lung. Which of the following is the most likely causal organism?
- A. Streptococcus pneumoniae (Correct Answer)
- B. Legionella pneumophila
- C. Pneumocystis jirovecii
- D. Staphylococcus aureus
- E. Cryptococcus neoformans
Microbiome functions and host defense Explanation: ***Streptococcus pneumoniae***
- This patient presents with **fever, chills, productive cough, dyspnea, leukocytosis with neutrophilia, and a lobar infiltrate on chest X-ray**, which are classic signs of **community-acquired bacterial pneumonia**.
- Although the patient is **HIV-positive**, his CD4+ count is >500/mm3 and he is on antiretroviral therapy, indicating relatively preserved immune function, making *S. pneumoniae* the most common cause of pneumonia even in HIV-infected individuals with controlled disease.
*Legionella pneumophila*
- While *Legionella* can cause pneumonia with fever and dyspnea, it is often associated with **gastrointestinal symptoms** (e.g., diarrhea) and **hyponatremia**, which are not present here.
- Exposure to contaminated water sources is a common risk factor, and the lobar infiltrate is less typical than diffuse or patchy infiltrates.
*Pneumocystis jirovecii*
- *Pneumocystis pneumonia (PJP)* is typically seen in **HIV patients with severely suppressed immune systems (CD4+ count <200/mm3)**.
- The patient's CD4+ count (520/mm3) is above this threshold, and PJP usually presents with diffuse interstitial infiltrates rather than a lobar infiltrate.
*Staphylococcus aureus*
- *S. aureus* pneumonia often occurs in the context of recent **influenza infection, intravenous drug use, or hospitalization**, or can present rapidly with **necrotizing pneumonia** or **empyema**.
- While possible, the absence of these specific risk factors or severe features makes it less likely than *S. pneumoniae* in this specific presentation.
*Cryptococcus neoformans*
- *Cryptococcus neoformans* is an opportunistic fungus that typically causes **pulmonary or central nervous system infections**, especially in severely immunocompromised patients (CD4+ count usually <100/mm3).
- Pulmonary cryptococcosis often manifests as **nodules or cavitary lesions**, or can be asymptomatic, which differs from the acute lobar pneumonia presented.
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