Infectious Diseases Practice Questions

Q: A 45-year-old man presents with diarrhea and abdominal pain. A biopsy reveals numerous macrophages filled with acid-fast organisms compressing lymphatic vessels. What is the most likely diagnosis?

Mycobacterium avium. ***Mycobacterium avium*** - The presence of **acid-fast organisms (AFO)** within macrophages compressing lymphatic vessels in a patient with diarrhea and abdominal pain is characteristic of **Mycobacterium avium complex (MAC)**. - MAC infection often causes widespread gastrointestinal involvement, particularly in immunocompromised individuals, leading to malabsorption and weight loss. *Giardia lamblia* - **Giardia lamblia** causes giardiasis, characterized by diarrhea, abdominal cramps, and malabsorption [2]. - However, giardia is a **flagellated protozoan** and would not appear as acid-fast organisms within macrophages [2]. *Mycobacterium tuberculosis* - While **Mycobacterium tuberculosis** is an acid-fast organism and can cause gastrointestinal disease, it typically presents with granulomas and caseous necrosis, rather than widespread macrophage infiltration with intact organisms compressing lymphatic vessels in this specific manner. - Gastrointestinal tuberculosis is less common than MAC in this presentation. *Entamoeba histolytica* - **Entamoeba histolytica** causes amebic dysentery, characterized by bloody diarrhea, and is associated with flask-shaped ulcers in the colon [1]. - It is a **protozoan parasite** and would not be identified as an acid-fast organism within macrophages [1].

Q: A 55-year-old male with diabetic neuropathy presents with a rapidly progressing necrotic lesion on his foot. Which organism is most likely responsible?

Clostridium perfringens. ***Clostridium perfringens*** - This organism is a common cause of **gas gangrene** (clostridial myonecrosis), which presents as rapidly progressing **necrotic lesions**, especially in compromised tissues like those found in diabetic neuropathy. - *C. perfringens* produces potent **toxins** that lead to rapid tissue destruction, gas formation, and systemic toxicity. *Staphylococcus aureus* - While *S. aureus* can cause severe skin and soft tissue infections, including cellulitis and abscesses, it typically does not cause the **rapidly progressing necrotic lesion** with gas production characteristic of gas gangrene. - Infections by *S. aureus* are often associated with **purulent drainage** but less with extensive tissue necrosis and gas in this context. *Streptococcus pyogenes* - *S. pyogenes* can cause **necrotizing fasciitis**, a rapidly progressing soft tissue infection, but it is typically characterized by widespread fascial necrosis with less emphasis on muscle necrosis and gas formation. - While severe, its presentation often lacks the overt **gas formation** seen with clostridial infections. *Pseudomonas aeruginosa* - *P. aeruginosa* is known to cause infections in compromised hosts, including diabetics, and can lead to **necrotic lesions** in severe cases. - However, it is more commonly associated with widespread cellulitis, osteomyelitis, or **hot tub folliculitis**, and rarely causes the fulminant, gas-producing necrosis seen with *C. perfringens*.

Q: A patient who recently returned from a hiking trip in the Rocky Mountains presents with fever, headache, and a rash on the wrists and ankles that is progressing to the trunk. What is the most likely diagnosis, and what are the key steps for confirmation of this diagnosis?

Rocky Mountain spotted fever. **Rocky Mountain spotted fever** - The classic triad of **fever, headache, and a centripetal rash** (starting on wrists/ankles and spreading to trunk) in a patient from an endemic area (Rocky Mountains) is highly characteristic of **Rocky Mountain spotted fever (RMSF)** [1], [2]. - Confirmation involves **serologic testing** (indirect immunofluorescence assay for IgG and IgM antibodies to *Rickettsia rickettsii*) and, in some cases, **PCR** of skin biopsy from the rash [2]. *Lyme disease* - While Lyme disease is also a tick-borne illness, its hallmark rash is **erythema migrans** (a target-like lesion), which is distinct from the described maculopapular to petechial rash of RMSF [3]. - Neurological, cardiac, and arthritic manifestations develop later in Lyme disease, and the acute presentation differs significantly. *Ehrlichiosis* - Ehrlichiosis typically presents with fever, headache, body aches, and fatigue, but a **rash is less common** (occurring in <10% of cases) and usually appears later in the course, without the characteristic centripetal progression of RMSF [3]. - Microscopic examination of peripheral blood smears may reveal **morulae within phagocytes**, a key diagnostic feature differentiating it from RMSF. *Anaplasmosis* - Anaplasmosis shares symptoms like fever, headache, and muscle aches but **rarely causes a rash** [3]. - Like ehrlichiosis, it can be diagnosed by finding **morulae within granulocytes** on a peripheral blood smear, which is not typical for RMSF [3].

Q: A patient presents with a productive cough and night sweats. Which diagnostic test would differentiate active pulmonary tuberculosis from other respiratory infections?

Sputum acid-fast bacillus stain. ### Sputum acid-fast bacillus stain - The **acid-fast bacillus (AFB) stain** directly identifies the presence of *Mycobacterium tuberculosis*, the pathogen responsible for tuberculosis, based on its unique cell wall properties [1]. - A positive AFB smear strongly suggests active TB, especially in a patient with **productive cough** and **night sweats** [1], [2]. *Chest X-ray* - A chest X-ray can show **pulmonary infiltrates**, **cavitations**, or **lymphadenopathy** consistent with TB, but these findings are not specific and can be seen in other respiratory infections or cancers [1]. - It aids in screening and monitoring disease progression but cannot definitively diagnose active TB or differentiate it from other infections. *Complete blood count* - A **complete blood count (CBC)** may reveal non-specific changes like **leukocytosis** or anemia of chronic disease, which are common in many infections and inflammatory conditions [3]. - It does not offer any specific diagnostic information for tuberculosis or help differentiate it from other respiratory infections. *Serum antibody test* - **Serum antibody tests** for TB have **low sensitivity and specificity**, especially for active pulmonary disease, and are not recommended for diagnosing active TB. - They primarily detect an immune response to *Mycobacterium tuberculosis* but cannot distinguish between active infection, latent infection, or past exposure reliably.

Q: A patient with no significant past medical history presents with peptic ulcer disease. Testing reveals a Helicobacter pylori infection. Evaluate the recommended treatment options while considering current resistance patterns.

Bismuth quadruple therapy. Bismuth quadruple therapy - This regimen is highly effective, especially in regions with high clarithromycin and metronidazole resistance, making it a preferred first-line or salvage therapy. - It combines a proton pump inhibitor (PPI), bismuth subsalicylate (or subcitrate), tetracycline, and metronidazole for 10-14 days. Levofloxacin-based therapy - This is typically reserved for salvage therapy after failure of bismuth quadruple or clarithromycin triple therapy, due to increasing fluoroquinolone resistance. - Its use as a first-line option is discouraged due to the potential for selecting for resistance and its broader spectrum effects. High-dose proton pump inhibitors - While PPIs are a critical component of H. pylori eradication regimens, using them alone is insufficient to eradicate the infection. - They primarily reduce acid production to promote ulcer healing, but they lack sufficient bactericidal activity against H. pylori on their own. Clarithromycin-based triple therapy - This regimen has a declining efficacy in many regions due to rising rates of clarithromycin resistance in H. pylori [1]. - It consists of a PPI, clarithromycin, and amoxicillin (or metronidazole), and is often no longer recommended as first-line in areas with high resistance (>15-20%) [1].

Q: A patient presents with severe abdominal pain and jaundice. Imaging reveals calcified cysts in the liver, and serological testing is positive for Echinococcus. What is a potential complication of this infection?

Secondary bacterial infection of cysts. ***Secondary bacterial infection of cysts*** - **Hydatid cysts** can become secondarily infected, typically after rupture or surgical manipulation, leading to an **abscess formation** within the liver. - This complication can present with features like fever, worsening abdominal pain, and an elevated **white blood cell count**, distinct from the initial presentation. *Rupture of cysts leading to anaphylactic shock* - While rupture of **hydatid cysts** can lead to **anaphylactic shock** due to the release of hydatid fluid, it is not the *most common immediate complication* or the scenario implied by abdominal pain and jaundice. - **Anaphylaxis** implies a rapid, severe systemic allergic reaction, which is a life-threatening acute event. *Portal hypertension due to cyst compression* - While large cysts can compress structures, significant **portal hypertension** due to direct compression of the portal vein by **hydatid cysts** is rare. - More commonly, portal hypertension is a complication of advanced **cirrhosis**, not typically direct cyst compression in the early stages described. *No significant complications* - **Echinococcosis** (hydatid disease) is a serious parasitic infection that almost always leads to significant morbidity if left untreated. - Cysts grow over time and inevitably cause **organ dysfunction**, pain, obstruction, or other complications.

Q: A 30-year-old patient presents with abdominal pain and jaundice. Serological tests are positive for anti-HCV antibodies and HCV RNA. What is the next step in management?

Initiate direct-acting antivirals. **Initiate direct-acting antivirals** - The presence of **anti-HCV antibodies** and **HCV RNA** indicates active **HCV infection**, warranting immediate antiviral treatment [1]. - **Direct-acting antivirals (DAAs)** are the current standard of care for chronic hepatitis C due to their high cure rates and favorable side effect profiles [1]. *Start interferon therapy* - **Interferon-based therapy** has largely been replaced by **DAAs** due to lower efficacy, significant side effects, and longer treatment durations for hepatitis C [1]. - It is now reserved for specific, rare circumstances where DAAs are contraindicated or unavailable. *Monitor liver enzymes* - While monitoring **liver enzymes** is part of hepatitis C management, simply observing them without active treatment for confirmed active infection would delay necessary intervention. - Monitoring is an ongoing process to assess disease progression and treatment response, not a first-line management strategy for active infection. *Perform liver biopsy* - **Liver biopsy** was historically used to stage liver fibrosis in hepatitis C, but non-invasive methods like **fibroScan** or **serum markers** are now preferred due to their safety and convenience. - It is not a necessary prerequisite for initiating antiviral therapy, especially given the clear evidence of active infection.

Q: In a patient with suspected tuberculosis, what is the most appropriate initial diagnostic test?

Chest X-ray. ***Chest X-ray*** - A **chest X-ray** is usually the **initial diagnostic test** for suspected active pulmonary tuberculosis (TB) to assess for lung involvement, such as **infiltrates, cavitations, or effusions** [1]. - While not definitive, it helps to **localize the disease** and can guide further diagnostic steps [1]. *Tuberculin skin test* - The **tuberculin skin test (TST)**, also known as the **Mantoux test**, primarily assesses for **latent TB infection** or previous exposure, not active disease, and cannot differentiate between the two. - A **positive TST** indicates an immune response to *Mycobacterium tuberculosis* but requires further investigation to rule out active disease. *Interferon-gamma release assay* - **Interferon-gamma release assays (IGRAs)**, like the TST, are used to diagnose **latent TB infection**, and cannot distinguish between latent and active TB [2]. - While **more specific** than TST, a positive IGRA still necessitates additional tests like a chest X-ray and sputum analysis to confirm active disease [2]. *Acid-fast bacillus (AFB) staining of sputum* - **AFB staining and microscopy** of sputum is a rapid method to identify active TB by detecting mycobacteria, providing presumptive evidence for diagnosis and guiding immediate isolation [1]. - However, it's typically performed **after an initial imaging study** like a chest X-ray suggests pulmonary involvement and before culture for definitive diagnosis [1].

Q: What is the treatment of choice for a patient with acute epiglottitis?

Intravenous antibiotics. ***Intravenous antibiotics*** - Acute epiglottitis is a rapidly progressive and potentially life-threatening bacterial infection, primarily affecting the **epiglottis** and surrounding structures. - **Intravenous antibiotics** are crucial for prompt treatment to prevent airway obstruction and systemic spread, as oral absorption may be delayed or insufficient. *Oral antibiotics* - **Oral antibiotics** are inappropriate for acute epiglottitis due to the acute and severe nature of the infection. - They may not achieve adequate bloodstream concentrations quickly enough to combat the rapid progression of inflammation and potential airway compromise. *Steroids* - While **corticosteroids** can help reduce inflammation, they are typically used as an adjunct to antibiotics, not as the primary treatment. - Steroids alone will not eradicate the bacterial infection responsible for epiglottitis. *Observation* - **Observation** is a dangerous approach for acute epiglottitis, as the condition can rapidly worsen and lead to **complete airway obstruction**. - Immediate medical and often airway management is necessary, not watchful waiting.

Q: A 25-year-old female presents with night sweats and fever. She has been previously treated for tuberculosis. Sputum examination reveals acid-fast bacilli (AFB). What category of treatment should be initiated?

Category II. ***Category II*** - This category is specifically designed for **retreatment cases** of tuberculosis, defined by a patient previously treated for TB who has now relapsed or failed treatment. Treatment failure is defined as a positive sputum smear or culture at 5 months [2]. - The presence of **night sweats**, **fever**, and **acid-fast bacilli (AFB)** [3] in sputum indicates active disease requiring reinstitution of a more intensive treatment regimen. *Category I* - This category is used for **newly diagnosed cases** of tuberculosis, [1] meaning patients who have never been treated for TB or have received less than one month of treatment. - The patient's history of **previous tuberculosis treatment** makes this category inappropriate. *Category III* - This category is typically reserved for **less severe forms of TB**, such as smear-negative pulmonary TB or non-severe extrapulmonary TB, in new patients. - The findings of **sputum AFB positivity** and a history of previous treatment rule out this category. *Category IV* - This category is for patients with **multidrug-resistant TB (MDR-TB)** or other forms of drug-resistant TB, requiring highly specialized and individualized treatment regimens. - While previous treatment failure might lead to drug resistance, the immediate indication from the given information is for a standard **retreatment regimen**, not necessarily MDR-TB directly.

Question 1

A 45-year-old man presents with diarrhea and abdominal pain. A biopsy reveals numerous macrophages filled with acid-fast organisms compressing lymphatic vessels. What is the most likely diagnosis?

Accepted Answer

Mycobacterium avium

Answer

Giardia lamblia

Answer

Mycobacterium tuberculosis

Answer

Entamoeba histolytica

Question 2

A 55-year-old male with diabetic neuropathy presents with a rapidly progressing necrotic lesion on his foot. Which organism is most likely responsible?

Accepted Answer

Clostridium perfringens

Answer

Staphylococcus aureus

Answer

Streptococcus pyogenes

Answer

Pseudomonas aeruginosa

Question 3

A patient who recently returned from a hiking trip in the Rocky Mountains presents with fever, headache, and a rash on the wrists and ankles that is progressing to the trunk. What is the most likely diagnosis, and what are the key steps for confirmation of this diagnosis?

Accepted Answer

Rocky Mountain spotted fever

Answer

Lyme disease

Answer

Ehrlichiosis

Answer

Anaplasmosis

Question 4

A patient presents with a productive cough and night sweats. Which diagnostic test would differentiate active pulmonary tuberculosis from other respiratory infections?

Accepted Answer

Sputum acid-fast bacillus stain

Answer

Chest X-ray

Answer

Complete blood count

Answer

Serum antibody test

Question 5

A patient with no significant past medical history presents with peptic ulcer disease. Testing reveals a Helicobacter pylori infection. Evaluate the recommended treatment options while considering current resistance patterns.

Accepted Answer

Bismuth quadruple therapy

Answer

Levofloxacin-based therapy

Answer

High-dose proton pump inhibitors

Answer

Clarithromycin-based triple therapy

Question 6

A patient presents with severe abdominal pain and jaundice. Imaging reveals calcified cysts in the liver, and serological testing is positive for Echinococcus. What is a potential complication of this infection?

Accepted Answer

Secondary bacterial infection of cysts

Answer

Portal hypertension due to cyst compression

Answer

Rupture of cysts leading to anaphylactic shock

Answer

No significant complications

Question 7

A 30-year-old patient presents with abdominal pain and jaundice. Serological tests are positive for anti-HCV antibodies and HCV RNA. What is the next step in management?

Accepted Answer

Initiate direct-acting antivirals

Answer

Start interferon therapy

Answer

Monitor liver enzymes

Answer

Perform liver biopsy

Question 8

In a patient with suspected tuberculosis, what is the most appropriate initial diagnostic test?

Accepted Answer

Chest X-ray

Answer

Tuberculin skin test

Answer

Interferon-gamma release assay

Answer

Acid-fast bacillus (AFB) staining of sputum

Question 9

What is the treatment of choice for a patient with acute epiglottitis?

Accepted Answer

Intravenous antibiotics

Answer

Oral antibiotics

Answer

Steroids

Answer

Observation

Question 10

A 25-year-old female presents with night sweats and fever. She has been previously treated for tuberculosis. Sputum examination reveals acid-fast bacilli (AFB). What category of treatment should be initiated?

Accepted Answer

Category II

Answer

Category I

Answer

Category III

Answer

Category IV

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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