Bacterial metabolism US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Bacterial metabolism. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Bacterial metabolism US Medical PG Question 1: A medical technician is trying to isolate a pathogen from the sputum sample of a patient. The sample is heat fixed to a slide then covered with carbol fuchsin stain and heated again. After washing off the stain with clean water, the slide is covered with 1% sulfuric acid for decolorization. The sample is rinsed again and stained with methylene blue. Microscopic examination shows numerous red, branching filamentous organisms. Which of the following is the most likely isolated pathogen?
- A. Cryptococcus neoformans
- B. Tropheryma whipplei
- C. Nocardia asteroides (Correct Answer)
- D. Rickettsia rickettsii
- E. Staphylococcus aureus
Bacterial metabolism Explanation: ***Nocardia asteroides***
- The described staining procedure is a **modified acid-fast stain**, indicated by the use of **carbol fuchsin**, heating, and decolorization with **weak acid (1% sulfuric acid)**, followed by a counterstain with methylene blue.
- **Nocardia species** are **weakly acid-fast bacteria** that resist decolorization with weak acids (1-3% sulfuric acid), appearing as **red, branching filamentous organisms** under this staining method.
- The **modified acid-fast stain** uses weaker decolorizing agents compared to the standard Ziehl-Neelsen stain, making it suitable for detecting weakly acid-fast organisms like Nocardia.
- Nocardia are aerobic actinomycetes commonly found in soil and can cause pulmonary infections, especially in immunocompromised patients.
*Cryptococcus neoformans*
- This is a **yeast** that is typically identified using an **India ink stain** to visualize its polysaccharide capsule, or through fungal stains like Gomori methenamine silver (GMS).
- It would not appear as acid-fast red branching filaments with the described technique.
*Tropheryma whipplei*
- This bacterium is typically identified by **periodic acid-Schiff (PAS) stain** in tissue biopsies, which highlights its cell wall glycoproteins (appears magenta).
- It is not acid-fast and would not retain the carbol fuchsin after acid decolorization.
*Rickettsia rickettsii*
- This is an **obligate intracellular bacterium** that is difficult to culture and is often diagnosed by **serological tests** or **immunohistochemistry** on skin biopsy specimens.
- It is not acid-fast and would not be detected by this staining technique.
*Staphylococcus aureus*
- This is a **Gram-positive coccus** that would be stained **purple** by a Gram stain as it retains crystal violet.
- It is not acid-fast and would be completely decolorized by sulfuric acid in the described procedure, appearing blue (counterstain color) rather than red.
Bacterial metabolism US Medical PG Question 2: A 26-year-old female presents to her primary care physician concerned that she has contracted a sexually transmitted disease. She states that she is having severe pain whenever she urinates and seems to be urinating more frequently than normal. She reports that her symptoms started after she began having unprotected sexual intercourse with 1 partner earlier this week. The physician obtains a urinalysis which demonstrates the following, SG: 1.010, Leukocyte esterase: Positive, Nitrites: Positive, Protein: Trace, pH: 5.0, RBC: Negative. A urease test is performed which is negative. This patient has most likely been infected with which of the following organisms?
- A. Enterobacter cloacae
- B. Staphylococcus saprophyticus
- C. Proteus mirabilis
- D. Klebsiella pneumoniae
- E. Escherichia coli (Correct Answer)
Bacterial metabolism Explanation: ***Escherichia coli***
- The urinalysis findings of **positive leukocyte esterase**, **nitrites**, and **trace protein** with a slightly acidic pH (5.0) are highly suggestive of a **urinary tract infection (UTI)**.
- *E. coli* is the most common cause of UTIs, especially in young, sexually active women, and is typically **urease-negative**, consistent with the information provided.
- *E. coli* accounts for **80-90% of uncomplicated UTIs** and produces nitrites from dietary nitrates, making it the most likely pathogen in this clinical scenario.
*Enterobacter cloacae*
- While *Enterobacter cloacae* can cause UTIs, it is less common than *E. coli* in uncomplicated cases and is often associated with nosocomial infections or those in immunocompromised individuals.
- Its urease activity can vary, so a negative urease test doesn't rule it out completely but makes *E. coli* a more likely primary choice in this context.
*Staphylococcus saprophyticus*
- *S. saprophyticus* is a common cause of UTIs in young, sexually active women (second most common cause after *E. coli*) and is typically **urease-negative**, which is consistent with the negative test.
- However, the presence of **positive nitrites** points more strongly towards **Gram-negative bacteria** like *E. coli*, as *S. saprophyticus* is a **Gram-positive coccus** that does not produce nitrite reductase and therefore does not convert nitrates to nitrites.
*Proteus mirabilis*
- *Proteus mirabilis* is known for causing UTIs and is characteristically **urease-positive**, leading to alkaline urine (higher pH) and sometimes **struvite stones**.
- The **negative urease test** and acidic urine pH (5.0) in this case effectively rule out *Proteus mirabilis*.
*Klebsiella pneumoniae*
- *Klebsiella pneumoniae* can cause UTIs and is generally **urease-negative**, but it is less frequently the cause of uncomplicated UTIs compared to *E. coli*.
- Although it can produce nitrites, *E. coli* remains the most common etiology in this clinical scenario.
Bacterial metabolism US Medical PG Question 3: A 16-year-old boy presents with acute left-sided weakness. The patient is obtunded and can not provide any history other than his stomach hurts. The patient’s friend states that the patient has had episodes like this in the past and that “he has the same weird disease as his mom”. On physical examination, strength is 1 out of 5 in the left upper and lower extremities. A noncontrast CT scan of the head is normal. Laboratory tests reveal an anion gap metabolic acidosis. Which of the following is a normal function of the structure causing this patient’s condition?
- A. Regulation of blood glucose
- B. Synthesis of fatty acids
- C. Production of bile acids
- D. Metabolism of purines
- E. Conversion of ammonia to urea (Correct Answer)
Bacterial metabolism Explanation: ***Conversion of ammonia to urea***
- The patient's presentation with **acute neurological deficits**, **abdominal pain**, and **anion gap metabolic acidosis** with a normal head CT, along with family history, is highly suggestive of a **urea cycle disorder (UCD)**.
- The **urea cycle** primarily functions in the **liver** to convert **toxic ammonia** into less toxic urea for excretion.
*Regulation of blood glucose*
- While regulation of blood glucose is a vital **liver function**, it does not directly relate to the primary metabolic derangement (ammonia accumulation) seen in UCDs.
- **Hypoglycemia** can occur in some UCDs, but it's not the defining feature of the neurological crisis.
*Synthesis of fatty acids*
- **Fatty acid synthesis** primarily occurs in the liver and adipose tissue, but its dysfunction is not the direct cause of the patient's acute symptoms.
- While liver dysfunction can impact lipid metabolism, it is not the central pathological process in UCDs.
*Production of bile acids*
- **Bile acid production** is a critical function of the liver for fat digestion and absorption, but it is not directly impaired in urea cycle disorders.
- Bile acid synthesis disorders would present with different clinical features, such as **cholestasis** and fat malabsorption.
*Metabolism of purines*
- **Purine metabolism** occurs in various tissues, and its dysfunction can lead to conditions like **gout** or **Lesch-Nyhan syndrome**, which differ from the presented symptoms.
- Abnormalities in purine metabolism are not the core defect in urea cycle disorders.
Bacterial metabolism US Medical PG Question 4: E. coli has the ability to regulate its enzymes to break down various sources of energy when available. It prevents waste by the use of the lac operon, which encodes a polycistronic transcript. At a low concentration of glucose and absence of lactose, which of the following occurs?
- A. Decreased cAMP levels result in poor binding to the catabolite activator protein
- B. Increased cAMP levels result in binding to the catabolite activator protein (Correct Answer)
- C. Increased allolactose levels bind to the repressor
- D. Repressor releases from lac operator
- E. Transcription of the lac Z, Y, and A genes increase
Bacterial metabolism Explanation: ***Increased cAMP levels result in binding to the catabolite activator protein***
- In the absence of glucose, **adenylate cyclase** activity increases, leading to higher levels of **cAMP**.
- **cAMP** then binds to the **catabolite activator protein (CAP)**, forming the **cAMP-CAP complex**, which is crucial for activating lac operon transcription in the absence of glucose.
*Decreased cAMP levels result in poor binding to the catabolite activator protein*
- **Decreased glucose levels** actually lead to **increased cAMP** synthesis, not decreased.
- High **cAMP** levels enhance, not hinder, its binding to **CAP**.
*Increased allolactose levels bind to the repressor*
- **Allolactose** is an inducer that forms in the presence of **lactose**, which is stated to be absent in this scenario.
- Therefore, **allolactose levels** would be low, and it would not bind to the **repressor**.
*Repressor releases from lac operator*
- The **repressor protein** is bound to the **lac operator** in the absence of lactose.
- For the **repressor to be released**, **allolactose** (formed from lactose) must be present to bind to it.
*Transcription of the lac Z, Y, and A genes increase*
- While **cAMP-CAP binding** would promote transcription, the **absence of lactose** means the **repressor remains bound** to the operator.
- This binding effectively blocks RNA polymerase, preventing significant transcription of the **lac Z, Y, and A genes**, regardless of high **cAMP** levels.
Bacterial metabolism US Medical PG Question 5: A 24-year-old woman presents to her primary care physician with a 3 day history of pain with urination. She says that this pain has been accompanied by abdominal pain as well as a feeling like she always needs to use the restroom. She has no past medical history and no family history that she can recall. She is currently sexually active with a new partner but has tested negative for sexually transmitted infections. Physical exam reveals suprapubic tenderness and urine culture reveals gram-positive cocci. Which of the following best describes the organism that is most likely causing this patient's symptoms?
- A. Catalase negative and beta-hemolytic
- B. Catalase negative and alpha-hemolytic
- C. Catalase positive and coagulase positive
- D. Coagulase negative and novobiocin sensitive
- E. Coagulase negative and novobiocin resistant (Correct Answer)
Bacterial metabolism Explanation: ***Coagulase negative and novobiocin resistant***
- The patient's symptoms (dysuria, abdominal pain, urinary urgency) combined with gram-positive cocci in the urine culture, especially in a sexually active young woman, are highly suggestive of a **Staphylococcus saprophyticus** urinary tract infection (UTI).
- *Staphylococcus saprophyticus* is characteristically **coagulase-negative** and naturally **resistant to novobiocin**.
*Catalase negative and beta-hemolytic*
- This describes organisms like **Streptococcus pyogenes**, which typically causes pharyngitis or skin infections, not UTIs with these characteristics.
- While *Streptococcus pyogenes* is gram-positive cocci, it is **catalase-negative**, differentiating it from *Staphylococcus* species.
*Catalase negative and alpha-hemolytic*
- This description fits **Streptococcus pneumoniae** or **viridans streptococci**.
- These organisms are generally associated with pneumonia, otitis media, or endocarditis, not commonly with UTIs presenting in this manner.
*Catalase positive and coagulase positive*
- This describes **Staphylococcus aureus**, which is a significant pathogen but less likely to cause uncomplicated UTIs in this demographic and presentation.
- While *Staphylococcus aureus* can cause UTIs, it's typically associated with a different clinical context or more severe infections.
*Coagulase negative and novobiocin sensitive*
- This describes **Staphylococcus epidermidis** and other common coagulase-negative staphylococci.
- While present on the skin, *Staphylococcus epidermidis* is usually a contaminant in urine cultures or causes UTIs in catheterized patients, and it is **novobiocin sensitive**, unlike *Staphylococcus saprophyticus*.
Bacterial metabolism US Medical PG Question 6: While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?
- A. Quellung reaction (Correct Answer)
- B. Hemolytic reaction when grown on sheep blood agar
- C. Bile solubility
- D. Optochin sensitivity
- E. Motility
Bacterial metabolism Explanation: ***Quellung reaction***
- The **Quellung reaction** tests for the presence of the **polysaccharide capsule**, which is the primary virulence factor of *S. pneumoniae*.
- An **avirulent strain** that cannot cause disease would most likely lack the capsule and show a **negative Quellung reaction** (no capsular swelling), deviating from the **positive reaction** seen in typical encapsulated pathogenic strains.
- The capsule enables *S. pneumoniae* to evade phagocytosis and complement-mediated killing, which is essential for establishing infection in the lungs.
*Hemolytic reaction when grown on sheep blood agar*
- Both virulent and avirulent strains of *S. pneumoniae* typically exhibit **alpha-hemolysis** (partial hemolysis, producing a greenish discoloration) on sheep blood agar due to the production of pneumolysin.
- This characteristic does not differentiate between pathogenic and non-pathogenic strains in terms of disease-causing ability.
*Bile solubility*
- *S. pneumoniae* is characteristically **bile-soluble** due to the presence of autolysin enzymes that are activated by bile salts, leading to cellular lysis.
- This property is a **species characteristic** present in both virulent and avirulent strains, thus it would not explain the inability to cause disease.
*Optochin sensitivity*
- *S. pneumoniae* is universally **sensitive to optochin**, a chemical agent that inhibits its growth and is used for laboratory identification.
- This characteristic is used for **species identification** but does not correlate with strain virulence or disease-causing ability.
*Motility*
- *Streptococcus pneumoniae* is a **non-motile** bacterium; it lacks flagella.
- This characteristic is consistent across all strains and is not a virulence factor for this species.
Bacterial metabolism US Medical PG Question 7: A 24-year-old man presents with low-grade fever and shortness of breath for the last 3 weeks. Past medical history is significant for severe mitral regurgitation status post mitral valve replacement five years ago. His temperature is 38.3°C (101.0°F) and respiratory rate is 18/min. Physical examination reveals vertical hemorrhages under his nails, multiple painless erythematous lesions on his palms, and two tender, raised nodules on his fingers. Cardiac auscultation reveals a new-onset 2/6 holosystolic murmur loudest at the apex with the patient in the left lateral decubitus position. A transesophageal echocardiogram reveals vegetations on the prosthetic valve. Blood cultures reveal catalase-positive, gram-positive cocci. Which of the following characteristics is associated with the organism most likely responsible for this patient’s condition?
- A. Coagulase positive
- B. DNAse positive
- C. Hemolysis
- D. Novobiocin sensitive (Correct Answer)
- E. Optochin sensitive
Bacterial metabolism Explanation: ***Novobiocin sensitive***
- The patient has **prosthetic valve endocarditis** caused by a **catalase-positive, gram-positive coccus**, which is most likely **_Staphylococcus epidermidis_** due to its association with foreign bodies and prosthetic devices.
- _Staphylococcus epidermidis_ is a **coagulase-negative staphylococcus** that is **novobiocin sensitive**, helping to differentiate it from other coagulase-negative staphylococci like **_Staphylococcus saprophyticus_** (novobiocin resistant).
- Although this is late prosthetic valve endocarditis (5 years post-surgery), _S. epidermidis_ remains a common pathogen due to biofilm formation on prosthetic materials.
*Coagulase positive*
- **Coagulase-positive** gram-positive cocci, such as **_Staphylococcus aureus_**, are a common cause of endocarditis, especially in intravenous drug users and can also cause prosthetic valve endocarditis.
- However, the correct answer requires identifying the characteristic that differentiates the most likely organism, and **coagulase-negative** staphylococci like _S. epidermidis_ are more characteristically associated with prosthetic device infections due to their biofilm-forming capabilities.
- A positive coagulase test differentiates _S. aureus_ from coagulase-negative staphylococci.
*DNAse positive*
- **DNAse positivity** is characteristic of **_Staphylococcus aureus_** and group A beta-hemolytic streptococci (_Streptococcus pyogenes_).
- While _S. aureus_ can cause prosthetic valve endocarditis, the question asks for the characteristic most associated with the likely organism, which in the context of prosthetic devices is typically **_S. epidermidis_** (DNAse negative).
*Hemolysis*
- **Hemolysis patterns** are primarily used to differentiate **streptococcal species**, not staphylococci. For example, **beta-hemolytic streptococci** cause complete hemolysis.
- While some staphylococci can show hemolytic activity, it is not a primary characteristic used to differentiate between the most likely staphylococcal causes of prosthetic valve endocarditis.
*Optochin sensitive*
- **Optochin sensitivity** is a key characteristic used to identify **_Streptococcus pneumoniae_**.
- _S. pneumoniae_ is **catalase-negative**, while the described organism is **catalase-positive**, ruling out _S. pneumoniae_ as the causative agent.
Bacterial metabolism US Medical PG Question 8: A 16-year-old girl presents to her physician with itching, soreness, and irritation in the vulvar region. She reports that these episodes have occurred 6–7 times a year since the age of 5. She used to treat these symptoms with topical ketoconazole cream, but this time it failed to help. She also has had several episodes of oral candidiasis in the past. She is not sexually active and does not take any medication. Her vital signs are as follows: the blood pressure is 115/80 mm Hg, the heart rate is 78/min, the respiratory rate is 15/min, and the temperature is 35.5°C (97.7°F). Examination shows vulvovaginal erythema with cottage cheese-like plaques and an intact hymen. Wet mount microscopy is positive for yeast. Along with a swab culture, the physician orders a dihydrorhodamine test and myeloperoxidase staining for a suspected primary immunodeficiency. The dihydrorhodamine test is positive, and the myeloperoxidase staining reveals diminished staining. Which of the following best describes this patient's condition?
- A. The patient is likely to have another immune impairment besides the one for which she was tested. (Correct Answer)
- B. The patient should receive prophylactic courses of wide spectrum antibiotics to prevent infections.
- C. The patient’s phagocytes are unable to generate an oxidative burst to kill intracellular bacteria.
- D. The patient is susceptible to all mycotic infections.
- E. The patient’s phagocytes can only perform extracellular killing.
Bacterial metabolism Explanation: ***The patient is likely to have another immune impairment besides the one for which she was tested.***
- The **positive dihydrorhodamine (DHR) test** indicates that the patient's phagocytes are capable of producing an **oxidative burst**, effectively ruling out **Chronic Granulomatous Disease (CGD)**.
- The **diminished myeloperoxidase (MPO) staining** suggests **Myeloperoxidase Deficiency**, which is usually asymptomatic in most patients but can predispose to recurrent Candida infections, especially in diabetics.
- However, **MPO deficiency alone does not explain** the severity and frequency of this patient's recurrent **oral and vulvovaginal candidiasis** starting from age 5, suggesting an additional immune defect.
- The pattern of chronic mucocutaneous candidiasis suggests a defect in **T-cell immunity** or the **CARD9 pathway**, which is crucial for antifungal responses against *Candida* species.
- Therefore, the patient likely has a **combined immunodeficiency**: MPO deficiency (detected) plus another defect affecting cell-mediated immunity or antifungal responses (not yet tested for).
*The patient should receive prophylactic courses of wide spectrum antibiotics to prevent infections.*
- The **positive DHR test** indicates normal oxidative burst, meaning the patient is not primarily susceptible to catalase-positive bacterial infections that would require prophylactic antibiotics (unlike in CGD).
- The recurrent infections are **fungal** (Candida), not bacterial, so broad-spectrum antibiotics would not be the appropriate prophylactic treatment.
- Appropriate management would include **antifungal prophylaxis** (e.g., fluconazole) and further investigation for T-cell defects.
*The patient's phagocytes are unable to generate an oxidative burst to kill intracellular bacteria.*
- This statement is **directly contradicted** by the **positive DHR test**, which demonstrates that phagocytes *are* capable of generating an oxidative burst.
- Inability to generate an oxidative burst is the hallmark of **Chronic Granulomatous Disease (CGD)**, where the DHR test would be **negative** (abnormal).
- In CGD, patients present with recurrent catalase-positive bacterial infections (Staphylococcus, Serratia, Nocardia) and invasive fungal infections (Aspergillus), not primarily mucocutaneous candidiasis.
*The patient is susceptible to all mycotic infections.*
- While the patient has **recurrent candidiasis**, there is no evidence of susceptibility to a broad range of other fungal pathogens (e.g., Aspergillus, Cryptococcus, Histoplasma).
- The specific pattern of **chronic mucocutaneous candidiasis** suggests a targeted defect in anti-Candida immunity (T-cell or CARD9 deficiency), rather than global susceptibility to all fungi.
- Susceptibility to all mycotic infections would be seen in severe combined immunodeficiencies (SCID) or advanced HIV/AIDS, which would present with multiple opportunistic infections.
*The patient's phagocytes can only perform extracellular killing.*
- This statement is **incorrect** because phagocytes (neutrophils and macrophages) primarily perform **intracellular killing** after engulfing pathogens through phagocytosis.
- The **positive DHR test** confirms intact oxidative burst, which is essential for **intracellular killing** of phagocytosed organisms.
- MPO deficiency affects the efficiency of intracellular killing (as MPO enhances the microbicidal activity of hydrogen peroxide), but phagocytes still retain other intracellular killing mechanisms (lysozyme, defensins, proteases).
Bacterial metabolism US Medical PG Question 9: A 45-year-old man is brought to the emergency department after being found down outside of a bar. He does not have any identifying information and is difficult to arouse. On presentation, his temperature is 101.2°F (38.4°C), blood pressure is 109/72 mmHg, pulse is 102/min, and respirations are 18/min. Physical exam reveals an ill-appearing and disheveled man with labored breathing and coughing productive of viscous red sputum. Lung auscultation demonstrates consolidation of the left upper lobe of the patient. Given these findings, cultures are obtained and broad spectrum antibiotics are administered. Which of the following agar types should be used to culture the most likely organism in this case?
- A. Charcoal yeast extract agar
- B. Blood agar
- C. MacConkey agar (Correct Answer)
- D. Eaton agar
- E. Löwenstein-Jensen agar
Bacterial metabolism Explanation: ***MacConkey agar***
- The patient's presentation with **viscous red "currant jelly" sputum**, **upper lobe consolidation**, and history of **alcohol use** is classic for pneumonia caused by ***Klebsiella pneumoniae***.
- **MacConkey agar** is a selective and differential medium used to isolate **gram-negative bacteria** such as *Klebsiella pneumoniae*, which appears as mucoid, lactose-fermenting (pink) colonies.
- The "currant jelly sputum" is pathognomonic for *Klebsiella* and distinguishes it from other pneumonias.
*Blood agar*
- While blood agar is a rich, non-selective medium that supports growth of many organisms including *Streptococcus pneumoniae*, it is not the most appropriate choice for this case.
- *Streptococcus pneumoniae* typically causes **rust-colored sputum**, not the viscous red sputum described here.
- Though *Klebsiella* can grow on blood agar, **MacConkey agar** is more specific for identifying gram-negative organisms like *Klebsiella*.
*Charcoal yeast extract agar*
- This medium is specifically designed for the isolation of **Legionella species**, which are fastidious gram-negative rods.
- *Legionella* pneumonia typically presents with relative bradycardia, hyponatremia, and diarrhea, not the viscous red sputum characteristic of *Klebsiella*.
*Löwenstein-Jensen agar*
- This specialized medium is used for the isolation of **mycobacteria**, particularly *Mycobacterium tuberculosis*.
- TB typically presents with chronic symptoms (weeks to months), night sweats, and hemoptysis, not the acute presentation with viscous red sputum seen here.
*Eaton agar*
- This specialized medium is used for the isolation of **Mycoplasma pneumoniae**, which causes "walking pneumonia."
- *Mycoplasma* pneumonia is typically mild with dry cough and patchy infiltrates, not the severe lobar consolidation and viscous red sputum seen in this case.
Bacterial metabolism US Medical PG Question 10: A 9-year-old girl comes to the clinic with a chief complaint of a swollen eye and sinus infection for 4 days. She complained of left nasal pain prior to these symptoms. The patient noticed that the swelling and redness of her left eye has progressively worsened. It has been difficult to open her eyelids, and she complains of diplopia and pain during ocular movement. The visual acuity is 20/20 in both eyes. Intraocular pressure measurement shows values of 23 and 14 mm Hg in the right and left eyes, respectively. The test results for the complete blood count, ESR, and CRP are as follows (on admission):
CBC results
Leukocytes 18,000 cells/mm3
Neutrophils 80%
Lymphocytes 14%
Eosinophils 1%
Basophils 0%
Monocytes 5%
Hemoglobin 12 g/dL
ESR 65
CRP 4.6
The organism causing the above condition is destroyed by which one of the following immunological processes?
- A. Perforins and granzymes by natural killer cells
- B. Release of cytotoxic granules by cytotoxic T cells
- C. Phagolysosome formation by neutrophils (Correct Answer)
- D. Activation of cytosolic caspases
- E. Complement-mediated membrane attack complex formation
Bacterial metabolism Explanation: ***Phagolysosome formation by neutrophils***
- The patient presents with symptoms highly suggestive of **orbital cellulitis**, evidenced by a swollen eye, pain with ocular movement, diplopia, and a history of sinus infection. The laboratory findings, including **leukocytosis (18,000 cells/mm3)** with a high percentage of **neutrophils (80%)**, and elevated **ESR (65)** and **CRP (4.6)**, indicate an active **bacterial infection**.
- **Neutrophils** are the primary immune cells responding to bacterial infections. Their main mechanism of destroying bacteria involves **phagocytosis**, where they engulf the pathogen and fuse the phagosome with lysosomes to form a **phagolysosome**, leading to bacterial degradation through oxidative burst and enzymatic digestion.
*Perforins and granzymes by natural killer cells*
- **Natural killer (NK) cells** primarily target **virus-infected cells** and **tumor cells** by inducing apoptosis through the release of perforins and granzymes.
- This mechanism is not the primary way the immune system combats **bacterial infections** like the orbital cellulitis described.
*Release of cytotoxic granules by cytotoxic T cells*
- **Cytotoxic T lymphocytes (CTLs)**, or CD8+ T cells, are crucial for eliminating **intracellular pathogens (e.g., viruses)** and **cancer cells** by inducing apoptosis.
- While important for cell-mediated immunity, CTLs are not the main effector cells against the **extracellular bacteria** commonly causing orbital cellulitis.
*Activation of cytosolic caspases*
- **Caspases** are a family of proteases that play an essential role in programmed cell death, or **apoptosis**.
- While apoptosis is a part of immune regulation, the **direct destruction of extracellular bacterial pathogens** in an active infection is not mediated by the activation of cytosolic caspases within the pathogen or host cells as a primary defense mechanism.
*Complement-mediated membrane attack complex formation*
- The **membrane attack complex (MAC)** formed by complement proteins C5b-C9 creates pores in bacterial membranes, leading to lysis.
- While MAC is effective against some gram-negative bacteria, the encapsulated bacteria commonly causing orbital cellulitis (such as **Streptococcus pneumoniae** and **Staphylococcus aureus**) are relatively **resistant to MAC-mediated lysis** due to their thick peptidoglycan layer or capsules. The primary mechanism of destruction for these pathogens is **neutrophil-mediated phagocytosis** rather than complement-mediated lysis.
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