M. tuberculosis and non-tuberculous mycobacteria — MCQs

10 questions

For which of the following patients would you recommend prophylaxis against mycobacterium avium-intracellulare?

You are seeing a patient in clinic who recently started treatment for active tuberculosis. The patient is currently being treated with rifampin, isoniazid, pyrazinamide, and ethambutol. The patient is not used to taking medicines and is very concerned about side effects. Specifically regarding the carbohydrate polymerization inhibiting medication, which of the following is a known side effect?

A 28-year-old woman comes to the physician because of a two-month history of fatigue and low-grade fevers. Over the past 4 weeks, she has had increasing shortness of breath, a productive cough, and a 5.4-kg (11.9-lb) weight loss. Three months ago, the patient returned from a two-month trip to China. The patient appears thin. Her temperature is 37.9°C (100.2°F), pulse is 75/min, and blood pressure is 125/70 mm Hg. Examination shows lymphadenopathy of the anterior and posterior cervical chain. Rales are heard at the left lower lobe of the lung on auscultation. Laboratory studies show a leukocyte count of 11,300/mm3 and an erythrocyte sedimentation rate of 90 mm/h. An x-ray of the chest shows a patchy infiltrate in the left lower lobe and ipsilateral hilar enlargement. Microscopic examination of the sputum reveals acid-fast bacilli; polymerase chain reaction is positive. Sputum cultures are pending. After placing the patient in an airborne infection isolation room, which of the following is the most appropriate next step in management?

An infectious disease investigator is evaluating the diagnostic accuracy of a new interferon-gamma-based assay for diagnosing tuberculosis in patients who have previously received a Bacillus Calmette-Guérin (BCG) vaccine. Consenting participants with a history of BCG vaccination received an interferon-gamma assay and were subsequently evaluated for tuberculosis by sputum culture. Results of the study are summarized in the table below. Tuberculosis, confirmed by culture No tuberculosis Total Positive interferon-gamma assay 90 6 96 Negative interferon-gamma assay 10 194 204 Total 100 200 300 Based on these results, what is the sensitivity of the interferon-gamma-based assay for the diagnosis of tuberculosis in this study?

A 55-year-old woman comes to the physician because of fevers for 2 weeks. She works as a nurse and recently returned from a charity work trip to India, where she worked in a medically-underserved rural community. A tuberculin skin test 3 months ago prior to her trip showed an induration of 3 mm. Physical examination is unremarkable. An x-ray of the chest shows right-sided hilar lymphadenopathy. A sputum culture shows acid-fast bacilli. Which of the following immunologic processes most likely occurred first?

A 43-year-old man with a history of chronic alcoholism presents with a chronic cough and dyspnea. He says he traveled to Asia about 4 months ago and his symptoms started shortly after he returned. His temperature is 40.2°C (104.4°F) and pulse is 92/min. Physical examination reveals poor personal hygiene and a cough productive of foul blood-streaked sputum. Auscultation reveals decreased breath sounds on the right. A chest radiograph reveals an ill-defined circular lesion in the right middle lobe. Which of the following is true regarding this patient’s most likely diagnosis?

A 45-year-old man presents with a 2-week history of night sweats, cough, and a fever. Past medical history includes HIV infection diagnosed 10 years ago, managed with HAART. He says he hasn’t been compliant with his HAART therapy as prescribed because it is too expensive and he is currently unemployed without insurance. A chest radiograph is performed and reveals a cavity in the right upper lobe of his lung. Which of the following lung infections is most likely causing this patient’s symptoms?

A 16-year-old female presents to her pediatrician complaining of 2 weeks of fever and 1 week of swollen lumps in her left armpit. Upon examination of the left upper extremity, her physician notes the presence of a single papule which the patient claimed appeared one week ago. The patient started her first job at a pet store 2.5 weeks ago. Which of the following is the vector of transmission of the causative agent?

A 35-year-old man from Thailand presents with low-grade fever, chronic cough, and night sweats for 3 months. He describes the cough as productive and producing white sputum that is sometimes streaked with blood. He also says he has lost 10 lb in the last 3 months. Past medical history is unremarkable. The patient denies any smoking history, alcohol, or recreational drug use. The vital signs include blood pressure 115/75 mm Hg, heart rate 120/min, respiratory rate 20/min, and temperature 36.6℃ (97.8℉). On physical examination, the patient is ill-looking and thin with no pallor or jaundice. Cardiopulmonary auscultation reveals some fine crackles in the right upper lobe. A chest radiograph reveals a right upper lobe homogeneous density. Which of the following tests would be most helpful in making a definitive diagnosis of active infection in this patient?

Q10

A 46-year-old woman comes to the physician for a 6-month history of worsening bronchial asthma control. Before this issue began, she only used her salbutamol inhaler once a day. Now, she has to use it multiple times daily and also reports frequent nighttime awakening. Seven months ago, she moved to an apartment that is damp and has mold on some of the walls. The physician injects 0.1 mL of Candida albicans extract on the mid-volar surface of the right arm intradermally. After 48 hours there is a palpable induration of 17 mm. This reaction is most likely a result of release of which of the following substances?

M. tuberculosis and non-tuberculous mycobacteria US Medical PG Practice Questions and MCQs

Question 1: For which of the following patients would you recommend prophylaxis against mycobacterium avium-intracellulare?

A. 30-year old HIV positive male with CD4 count of 20 cells/microliter and a viral load of < 50 copies/mL (Correct Answer)
B. 22-year old HIV positive female with CD4 count of 750 cells/microliter and a viral load of 500,000 copies/mL
C. 45-year old HIV positive female with CD4 count of 250 cells/microliter and a viral load of 100,000 copies/mL
D. 50-year old HIV positive female with CD4 count of 150 cells/microliter and a viral load of < 50 copies/mL
E. 36-year old HIV positive male with CD4 count of 75 cells/microliter and an undetectable viral load

Explanation: ***30-year old HIV positive male with CD4 count of 20 cells/microliter and a viral load of < 50 copies/mL*** - Prophylaxis against **Mycobacterium avium complex (MAC)** is recommended for HIV-positive individuals with a **CD4 count below 50 cells/µL** to prevent disseminated MAC infection. - While an undetectable viral load suggests effective antiretroviral therapy (ART) in general, the extremely low CD4 count indicates severe immunosuppression, making prophylaxis crucial. *36-year old HIV positive male with CD4 count of 75 cells/microliter and an undetectable viral load* - The **CD4 count of 75 cells/µL** is above the threshold of 50 cells/µL for MAC prophylaxis, even though it's still low. - An **undetectable viral load** indicates successful ART, which generally helps improve immune function over time, albeit slowly in this CD4 range. *22-year old HIV positive female with CD4 count of 750 cells/microliter and a viral load of 500,000 copies/mL* - A **CD4 count of 750 cells/µL** is well above the threshold for MAC prophylaxis, indicating relatively preserved immune function. - Although the **viral load is very high**, suggesting uncontrolled HIV replication, the immune system is currently strong enough to ward off MAC. *45-year old HIV positive female with CD4 count of 250 cells/microliter and a viral load of 100,000 copies/mL* - A **CD4 count of 250 cells/µL** is above the threshold for MAC prophylaxis, which is 50 cells/µL. - While the **high viral load** implies an increased risk for opportunistic infections over time, other specific prophylaxes (e.g., PCP if <200) would be considered earlier. *50-year old HIV positive female with CD4 count of 150 cells/microliter and a viral load of < 50 copies/mL* - A **CD4 count of 150 cells/µL** is above the threshold for MAC prophylaxis (50 cells/µL). - An **undetectable viral load** is a positive sign of ART efficacy, but this patient would still require prophylaxis for **Pneumocystis jirovecii pneumonia (PCP)**, as her CD4 count is below 200 cells/µL.

Question 2: You are seeing a patient in clinic who recently started treatment for active tuberculosis. The patient is currently being treated with rifampin, isoniazid, pyrazinamide, and ethambutol. The patient is not used to taking medicines and is very concerned about side effects. Specifically regarding the carbohydrate polymerization inhibiting medication, which of the following is a known side effect?

A. Vision loss (Correct Answer)
B. Paresthesias of the hands and feet
C. Cutaneous flushing
D. Arthralgias
E. Elevated liver enzymes

Explanation: ***Vision loss*** - The "carbohydrate polymerization inhibiting medication" refers to **ethambutol**, which inhibits **arabinosyl transferase** (involved in mycobacterial cell wall arabinogalactan synthesis) - **Ethambutol** causes **optic neuritis**, leading to **decreased visual acuity**, **red-green color blindness**, and potentially **irreversible vision loss** - **Regular ophthalmologic monitoring** is essential during ethambutol therapy *Paresthesias of the hands and feet* - This describes **peripheral neuropathy** caused by **isoniazid** - Isoniazid interferes with **pyridoxine (vitamin B6) metabolism**, leading to neurotoxicity - Risk factors include malnutrition, diabetes, alcoholism, and pregnancy - Prevented by **pyridoxine supplementation** *Cutaneous flushing* - Not a characteristic side effect of first-line anti-tuberculosis medications - More commonly associated with niacin or certain allergic/vasodilatory reactions *Arthralgias* - Classic side effect of **pyrazinamide**, often affecting small joints - Caused by **pyrazinamide-induced hyperuricemia** (inhibits renal uric acid excretion) - May require dose adjustment or discontinuation if severe *Elevated liver enzymes* - **Hepatotoxicity** can occur with **rifampin**, **isoniazid**, and **pyrazinamide** - Requires regular monitoring of liver function tests during TB treatment - Most common serious adverse effect of combination TB therapy

Question 3: A 28-year-old woman comes to the physician because of a two-month history of fatigue and low-grade fevers. Over the past 4 weeks, she has had increasing shortness of breath, a productive cough, and a 5.4-kg (11.9-lb) weight loss. Three months ago, the patient returned from a two-month trip to China. The patient appears thin. Her temperature is 37.9°C (100.2°F), pulse is 75/min, and blood pressure is 125/70 mm Hg. Examination shows lymphadenopathy of the anterior and posterior cervical chain. Rales are heard at the left lower lobe of the lung on auscultation. Laboratory studies show a leukocyte count of 11,300/mm3 and an erythrocyte sedimentation rate of 90 mm/h. An x-ray of the chest shows a patchy infiltrate in the left lower lobe and ipsilateral hilar enlargement. Microscopic examination of the sputum reveals acid-fast bacilli; polymerase chain reaction is positive. Sputum cultures are pending. After placing the patient in an airborne infection isolation room, which of the following is the most appropriate next step in management?

A. Administer only isoniazid for 9 months
B. Administer isoniazid, rifampin, pyrazinamide, and ethambutol for 2 months, followed by isoniazid and rifampin for 4 months (Correct Answer)
C. Await culture results before initiating treatment
D. Obtain CT scan of the chest
E. Perform interferon-γ release assay

Explanation: ***Administer isoniazid, rifampin, pyrazinamide, and ethambutol for 2 months, followed by isoniazid and rifampin for 4 months*** - This patient presents with symptoms such as **fatigue, fever, weight loss, productive cough, and shortness of breath**, along with **lymphadenopathy, rales, patchy infiltrate, ipsilateral hilar enlargement, and acid-fast bacilli in sputum**, all highly suggestive of active **pulmonary tuberculosis (TB)**. - Given the strong clinical and microbiological evidence (acid-fast bacilli, positive PCR), immediate initiation of a **four-drug regimen (isoniazid, rifampin, pyrazinamide, ethambutol)** is crucial to prevent disease progression, reduce transmission, and ensure effective treatment. *Administer only isoniazid for 9 months* - **Isoniazid monotherapy** is typically used for **latent tuberculosis infection (LTBI)**, not active TB, as it is insufficient to treat active disease and carries a high risk of developing drug resistance. - The patient's symptoms, imaging findings, and positive acid-fast bacilli smear indicate **active disease**, not just latent infection. *Await culture results before initiating treatment* - Delaying treatment for active TB until **culture results** are available (which can take several weeks) is inappropriate and can lead to disease progression, increased morbidity, and higher risk of transmission to others. - The **acid-fast bacilli smear and PCR positivity** provide sufficient evidence to initiate empiric treatment immediately. *Obtain CT scan of the chest* - While a **CT scan** might provide more detailed imaging information, it is not the most immediate next step for management when active TB is strongly suspected and requires urgent treatment initiation. - The **chest x-ray findings** are already consistent with TB and sufficient to guide initial management. *Perform interferon-γ release assay* - An **interferon-γ release assay (IGRA)** is used to diagnose **latent tuberculosis infection (LTBI)**, not active TB, and does not differentiate between the two. - The patient's presentation with active symptoms, positive sputum smear, and positive PCR strongly indicates **active TB**, rendering an IGRA redundant and not helpful for determining the immediate treatment course.

Question 4: An infectious disease investigator is evaluating the diagnostic accuracy of a new interferon-gamma-based assay for diagnosing tuberculosis in patients who have previously received a Bacillus Calmette-Guérin (BCG) vaccine. Consenting participants with a history of BCG vaccination received an interferon-gamma assay and were subsequently evaluated for tuberculosis by sputum culture. Results of the study are summarized in the table below. Tuberculosis, confirmed by culture No tuberculosis Total Positive interferon-gamma assay 90 6 96 Negative interferon-gamma assay 10 194 204 Total 100 200 300 Based on these results, what is the sensitivity of the interferon-gamma-based assay for the diagnosis of tuberculosis in this study?

A. 90/96
B. 100/300
C. 194/200
D. 90/100 (Correct Answer)
E. 194/204

Explanation: ***90/100*** - **Sensitivity** measures the proportion of **true positive** cases that are correctly identified by the test. - In this study, there are 90 true positive results (positive interferon-gamma assay in patients with confirmed tuberculosis) out of a total of 100 individuals with confirmed tuberculosis (90 + 10). *90/96* - This calculation represents the **positive predictive value** (90 true positives / 96 total positive tests). - It answers the question: "If the test is positive, what is the likelihood that the patient actually has the disease?" *100/300* - This value represents the prevalence of tuberculosis in the study population (100 confirmed cases / 300 total participants). - It does not reflect a measure of the test's diagnostic accuracy. *194/200* - This value represents the **specificity** of the test (194 true negatives / 200 total individuals without tuberculosis). - Specificity measures the proportion of true negative cases that are correctly identified by the test. *194/204* - This calculation represents the **negative predictive value** (194 true negatives / 204 total negative tests). - It answers the question: "If the test is negative, what is the likelihood that the patient does not have the disease?"

Question 5: A 55-year-old woman comes to the physician because of fevers for 2 weeks. She works as a nurse and recently returned from a charity work trip to India, where she worked in a medically-underserved rural community. A tuberculin skin test 3 months ago prior to her trip showed an induration of 3 mm. Physical examination is unremarkable. An x-ray of the chest shows right-sided hilar lymphadenopathy. A sputum culture shows acid-fast bacilli. Which of the following immunologic processes most likely occurred first?

A. Production of interferon-gamma by T-helper cells
B. Migration of T-helper cells to the lungs
C. Replication of bacteria within alveolar macrophages (Correct Answer)
D. Formation of a nodular tubercle in the lung
E. Transportation of bacterial peptides to regional lymph nodes

Explanation: ***Replication of bacteria within alveolar macrophages*** - After initial infection, **Mycobacterium tuberculosis** is phagocytosed by **alveolar macrophages** in the lungs, where it **replicates unimpeded** for about 2–4 weeks before the adaptive immune response is fully mounted. - This phase of unchecked bacterial growth precedes the immune system's attempt to contain the infection, making it the first significant immunologic event. *Transportation of bacterial peptides to regional lymph nodes* - This process involves **antigen-presenting cells** (APCs) — typically macrophages or dendritic cells — migrating from the lungs to regional lymph nodes to present bacterial antigens to T cells. - This step occurs *after* the initial bacterial replication and phagocytosis but *before* a robust T-cell mediated immune response develops, as T-cells need to be activated in the lymph nodes. *Formation of a nodular tubercle in the lung* - The **tubercle** (granuloma) is a hallmark of tuberculosis, representing the body's attempt to contain the infection. - Its formation is a complex process involving activated macrophages, T cells, and fibroblasts, and it occurs *after* the initial bacterial replication and the subsequent immune cell activation and recruitment. *Migration of T-helper cells to the lungs* - **T-helper cells** migrate to the lungs only after they have been **activated** in the regional lymph nodes by antigen-presenting cells. - This migration is crucial for orchestrating the immune response and containing the infection but happens *after* initial bacterial proliferation and antigen presentation. *Production of interferon-gamma by T-helper cells* - **Interferon-gamma** (IFN-$\gamma$) is a key cytokine produced by activated T-helper cells (Th1 cells) that activates macrophages to become more effective at killing intracellular bacteria. - This production signifies a mature adaptive immune response and occurs *after* T-helper cell activation in the lymph nodes and subsequent migration to the infected site.

Question 6: A 43-year-old man with a history of chronic alcoholism presents with a chronic cough and dyspnea. He says he traveled to Asia about 4 months ago and his symptoms started shortly after he returned. His temperature is 40.2°C (104.4°F) and pulse is 92/min. Physical examination reveals poor personal hygiene and a cough productive of foul blood-streaked sputum. Auscultation reveals decreased breath sounds on the right. A chest radiograph reveals an ill-defined circular lesion in the right middle lobe. Which of the following is true regarding this patient’s most likely diagnosis?

A. A positive tuberculin test would be diagnostic of active infection.
B. DNA polymerase chain reaction (PCR) has poor sensitivity when applied to smear positive specimens.
C. Inoculation of a sputum sample into selective agar media needs to be incubated at 35–37°C (95.0–98.6°F) for up to 8 weeks. (Correct Answer)
D. Stains of gastric washing and urine have a high diagnostic yield on microscopy.
E. Ziehl-Neelsen staining is more sensitive than fluorescence microscopy with auramine-rhodamine stain.

Explanation: ***Inoculation of a sputum sample into selective agar media needs to be incubated at 35–37°C (95.0–98.6°F) for up to 8 weeks.*** - The patient's symptoms, travel history, imaging findings (ill-defined circular lesion), and social history (chronic alcoholism, poor hygiene) strongly suggest **pulmonary tuberculosis (TB)** with potential cavitation. - **Mycobacterium tuberculosis** is a slow-growing organism; therefore, **culture on selective media** (e.g., Lowenstein-Jensen or Middlebrook media) for an extended period (typically 6-8 weeks) is the gold standard for diagnosis, allowing for identification and drug susceptibility testing. *A positive tuberculin test would be diagnostic of active infection.* - A **positive tuberculin skin test (TST)** or **interferon-gamma release assay (IGRA)** indicates **TB exposure** or **latent TB infection**, not necessarily active disease. - Many individuals with latent TB will have a positive test but no active infection, requiring further diagnostic workup like sputum microscopy and culture to confirm active disease. *DNA polymerase chain reaction (PCR) has poor sensitivity when applied to smear positive specimens.* - **DNA PCR** is a rapid and highly **sensitive** and **specific** test for M. tuberculosis, especially valuable for **smear-positive specimens**. - It can detect very small amounts of mycobacterial DNA, significantly reducing the diagnostic time compared to culture. *Stains of gastric washing and urine have a high diagnostic yield on microscopy.* - While M. tuberculosis can be found in gastric washings, especially in patients who cannot produce sputum, its **diagnostic yield for pulmonary TB by microscopy is not high** compared to sputum. - **Urine microscopy** is rarely used for the diagnosis of pulmonary tuberculosis; it is only relevant in cases of **renal or genitourinary tuberculosis**. *Ziehl-Neelsen staining is more sensitive than fluorescence microscopy with auramine-rhodamine stain.* - **Fluorescence microscopy with auramine-rhodamine stain** is typically **more sensitive** than Ziehl-Neelsen staining for detecting acid-fast bacilli (AFB). - The fluorescent stain allows for lower magnification scanning and better visualization of the bacilli, leading to a higher detection rate.

Question 7: A 45-year-old man presents with a 2-week history of night sweats, cough, and a fever. Past medical history includes HIV infection diagnosed 10 years ago, managed with HAART. He says he hasn’t been compliant with his HAART therapy as prescribed because it is too expensive and he is currently unemployed without insurance. A chest radiograph is performed and reveals a cavity in the right upper lobe of his lung. Which of the following lung infections is most likely causing this patient’s symptoms?

A. M. tuberculosis (Correct Answer)
B. Cytomegalovirus
C. Mycobacterium avium complex
D. Pneumocystis jirovecii
E. Histoplasmosis

Explanation: ***M. tuberculosis*** - This patient's symptoms (night sweats, cough, fever), along with a cavitary lesion on chest X-ray, are classic for **pulmonary tuberculosis**, especially in an immunocompromised individual with HIV and likely poor HAART adherence. - **HIV infection** significantly increases the risk for reactivation of latent TB and progression to active disease. *Cytomegalovirus* - **CMV pneumonia** is rare in HIV patients unless their CD4 count is severely low and they are not on ART. It typically presents with dyspnea and hypoxemia, and imaging may show diffuse interstitial infiltrates rather than a cavitary lesion. - While CMV can cause **retinitis**, esophagitis, or colitis in HIV/AIDS patients, isolated cavitary lung lesions are not a typical presentation. *Mycobacterium avium complex* - **MAC infection** typically occurs in advanced HIV disease (CD4 < 50 cells/mm3) and usually presents as disseminated disease with symptoms like wasting, diarrhea, and anemia, rather than primarily cavitary lung lesions. - While pulmonary MAC can occur, it's less likely to present with a cavitary lesion than M. tuberculosis and would be in a more severely immunocompromised state. *Pneumocystis jirovecii* - **Pneumocystis pneumonia (PJP)** is a common opportunistic infection in HIV patients with low CD4 counts, presenting with dyspnea, non-productive cough, and hypoxemia. - Chest imaging typically shows **diffuse bilateral interstitial infiltrates**, not a cavitary lesion. *Histoplasmosis* - **Histoplasmosis** is more prevalent in specific geographic regions (e.g., Ohio and Mississippi River valleys) and can cause pulmonary infiltrates or disseminated disease in immunocompromised individuals. - While cavitary lesions can occur, the combination of night sweats, cough, and a cavitary lesion in an HIV patient points more strongly towards **tuberculosis** as the most common and classic presentation.

Question 8: A 16-year-old female presents to her pediatrician complaining of 2 weeks of fever and 1 week of swollen lumps in her left armpit. Upon examination of the left upper extremity, her physician notes the presence of a single papule which the patient claimed appeared one week ago. The patient started her first job at a pet store 2.5 weeks ago. Which of the following is the vector of transmission of the causative agent?

A. Cats (Correct Answer)
B. Rabbits
C. Animal urine
D. Parrots
E. Armadillos

Explanation: ***Cats*** - The combination of **fever**, **swollen lymph nodes** (lumps in the armpit), a **single papule**, and a recent history of working at a **pet store** strongly suggests **cat scratch disease**. - **Cat scratch disease** is caused by *Bartonella henselae*, which is primarily transmitted to humans through the **scratch or bite of a cat**, especially kittens. *Rabbits* - Rabbits are known vectors for diseases like **tularemia**, which can cause fever and swollen lymph nodes. - However, the typical presentation of tularemia often includes a more prominent **ulcerative lesion** at the site of inoculation, and the papule described is less characteristic. *Animal urine* - While animal urine can transmit diseases like **leptospirosis**, which can cause fever and various systemic symptoms, it typically does not present with a localized papule followed by regional lymphadenopathy in this manner. - Exposure to animal urine usually occurs through contact with contaminated water or soil, and the pet store context points more towards direct animal contact. *Parrots* - Parrots are associated with **psittacosis** (parrot fever), caused by *Chlamydia psittaci*. - Psittacosis primarily manifests as a **respiratory illness** (pneumonia) and does not typically present with a localized papule and regional lymphadenopathy. *Armadillos* - Armadillos are significant reservoirs for **Mycobacterium leprae**, the causative agent of **leprosy**. - Leprosy has a very long incubation period and presents with skin lesions, nerve damage, and sometimes lymphadenopathy, but a 2-week onset and the described acute symptoms are inconsistent with leprosy.

Question 9: A 35-year-old man from Thailand presents with low-grade fever, chronic cough, and night sweats for 3 months. He describes the cough as productive and producing white sputum that is sometimes streaked with blood. He also says he has lost 10 lb in the last 3 months. Past medical history is unremarkable. The patient denies any smoking history, alcohol, or recreational drug use. The vital signs include blood pressure 115/75 mm Hg, heart rate 120/min, respiratory rate 20/min, and temperature 36.6℃ (97.8℉). On physical examination, the patient is ill-looking and thin with no pallor or jaundice. Cardiopulmonary auscultation reveals some fine crackles in the right upper lobe. A chest radiograph reveals a right upper lobe homogeneous density. Which of the following tests would be most helpful in making a definitive diagnosis of active infection in this patient?

A. PPD test
B. Silver stain
C. Ziehl-Neelsen stain (Correct Answer)
D. Gram stain
E. Interferon-gamma assay

Explanation: ***Ziehl-Neelsen stain*** - The patient's symptoms (low-grade fever, chronic cough with white/bloody sputum, night sweats, weight loss) and chest X-ray findings (right upper lobe homogeneous density) are highly suggestive of **active tuberculosis (TB)**, especially given his origin from Thailand (a country with a high TB burden). - The **Ziehl-Neelsen stain** (acid-fast stain) directly visualizes **acid-fast bacilli** (AFB) like *Mycobacterium tuberculosis* in sputum, providing a rapid and definitive diagnosis of active infection. *PPD test* - A **PPD test** (tuberculin skin test) indicates exposure to *Mycobacterium tuberculosis* but cannot differentiate between **latent TB infection** and **active disease**. - A positive PPD can occur in individuals previously exposed or vaccinated with BCG, offering no direct evidence of current active infection. *Silver stain* - **Silver stain** (e.g., Gomori methenamine silver) is used to identify fungal organisms like *Pneumocystis jirovecii* or certain bacteria, such as *Legionella pneumophila*. - It is not the primary stain for diagnosing tuberculosis, which requires detection of acid-fast bacilli. *Gram stain* - **Gram stain** is used to classify bacteria based on their cell wall properties (Gram-positive or Gram-negative). - *Mycobacterium tuberculosis* has a unique **mycolic acid-rich cell wall** that makes it resistant to Gram staining and requires acid-fast staining for visualization. *Interferon-gamma assay* - An **interferon-gamma release assay (IGRA)**, like the Quantiferon-TB Gold test, detects exposure to *Mycobacterium tuberculosis* and is used to diagnose **latent TB infection**. - Similar to the PPD test, it cannot distinguish between latent infection and **active disease**, and a positive result requires further investigation for active TB.

Question 10: A 46-year-old woman comes to the physician for a 6-month history of worsening bronchial asthma control. Before this issue began, she only used her salbutamol inhaler once a day. Now, she has to use it multiple times daily and also reports frequent nighttime awakening. Seven months ago, she moved to an apartment that is damp and has mold on some of the walls. The physician injects 0.1 mL of Candida albicans extract on the mid-volar surface of the right arm intradermally. After 48 hours there is a palpable induration of 17 mm. This reaction is most likely a result of release of which of the following substances?

A. Interleukin-10
B. Superoxide anion
C. Tryptase
D. Interferon-γ (Correct Answer)
E. Lysozyme

Explanation: ***Interferon-γ*** - The patient's worsened asthma, fungal exposure, and positive delayed-type hypersensitivity (DTH) skin test to *Candida albicans* suggest a **Th1-mediated immune response**. - **Interferon-γ (IFN-γ)** is a key cytokine produced by Th1 cells, crucial for activating macrophages and cell-mediated immunity, which drives the induration observed in DTH reactions. *Interleukin-10* - **Interleukin-10 (IL-10)** is primarily an **anti-inflammatory cytokine** that suppresses immune responses, particularly Th1 and Th2 activity. - Its release is associated with downregulating, rather than mediating, the robust inflammatory reaction seen in a positive DTH test. *Superoxide anion* - **Superoxide anion** is a reactive oxygen species produced by phagocytes (e.g., neutrophils, macrophages) as part of the **respiratory burst** to kill ingested pathogens. - While important for host defense, it is not the primary mediator responsible for the induration and cellular infiltration characteristic of a *Candida* DTH skin test. *Tryptase* - **Tryptase** is an enzyme released by **mast cells** upon activation, typically during **immediate hypersensitivity reactions (Type I)**. - Its presence is indicative of allergic reactions mediated by IgE, which manifest as wheal and flare, not the delayed induration seen in this case. *Lysozyme* - **Lysozyme** is an enzyme found in secretions (e.g., tears, saliva) and phagocytes, which degrades bacterial cell walls. - It plays a role in innate immunity against bacteria but is not directly involved in the mediation of a delayed-type hypersensitivity reaction to fungal antigens.

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