Infectious Diseases Practice Questions

Q: A 25-year-old woman presents with fever, rash, and joint pain after returning from Southeast Asia. What is the most likely diagnosis?

Dengue fever. ***Dengue fever*** - The classic triad of **fever**, **rash**, and **arthralgia** (joint pain) following travel to endemic areas like Southeast Asia is highly suggestive of dengue [1]. - Dengue is a **mosquito-borne viral infection** common in tropical and subtropical regions. *Malaria* - Characterized by **cyclic fevers**, chills, and sweats [2], but rash and prominent joint pain are not typical features. - Diagnosed by **microscopic examination** of blood smears for parasites or rapid diagnostic tests. *Typhoid fever* - Presents with a **prolonged fever**, headache, abdominal pain, and often a **"rose spot" rash**, but significant joint pain is uncommon. - Caused by **Salmonella Typhi** and transmitted through contaminated food or water. *Leptospirosis* - Typically causes **fever**, headache, muscle pain (myalgia), and sometimes jaundice, but a widespread rash and prominent arthralgia are less characteristic. - Transmitted through contact with **urine of infected animals** or contaminated water.

Q: A patient presents with fever, headache, and a bull's-eye rash. What is the most likely diagnosis, and which vector is associated with this disease?

Lyme disease, Ixodes tick. ***Lyme disease, Ixodes tick*** - The classic triad of **fever**, **headache**, and a **bull's-eye rash** (erythema migrans) is highly characteristic of Lyme disease [1]. - **Lyme disease** is transmitted by the **Ixodes tick**, also known as the deer tick [1]. *Rocky Mountain spotted fever, Dermacentor tick* - Rocky Mountain spotted fever presents with a **maculopapular rash** that typically starts on the extremities and spreads centrally, often involving palms and soles, but not a bull's-eye pattern [2]. - It is transmitted by the **Dermacentor tick** (dog tick or wood tick). *Ehrlichiosis, Amblyomma tick* - Ehrlichiosis typically presents with **fever**, **headache**, **myalgia**, and thrombocytopenia, but usually no rash, or a non-specific rash, rather than a bull's-eye [1]. - It is primarily transmitted by the **Amblyomma americanum tick** (lone star tick). *Babesiosis, Ixodes tick* - Babesiosis can cause **fever**, **fatigue**, **malaise**, and **hemolytic anemia**, but it does not cause a bull's-eye rash. - While it is transmitted by the **Ixodes tick**, the clinical presentation is distinct from Lyme disease [1].

Q: Which test is used to diagnose active tuberculosis?

Sputum culture. ***Sputum culture*** - **Sputum culture** for *Mycobacterium tuberculosis* is considered the **gold standard** for diagnosing **active pulmonary tuberculosis** because it directly detects the presence of viable bacteria [2]. - A positive culture confirms active disease and allows for **drug susceptibility testing**, which is crucial for guiding treatment [1]. *Mantoux test* - The **Mantoux test** (tuberculin skin test) detects exposure to *Mycobacterium tuberculosis* and subsequent **immune sensitization**, indicating latent infection or past exposure, not necessarily active disease [3]. - It does not differentiate between **latent tuberculosis infection (LTBI)** and **active TB**, and a positive result requires further investigation to rule out active disease. *Chest X-ray* - A **chest X-ray** can show abnormalities consistent with tuberculosis (e.g., infiltrates, cavities, hilar lymphadenopathy), but it is not definitive for **active TB** [1]. - Abnormal findings on a chest X-ray must be correlated with clinical symptoms and often require **microbiological confirmation** to diagnose active disease. *ELISA* - **ELISA-based tests** are primarily used for detecting **antibodies** to *Mycobacterium tuberculosis* antigens, which can indicate exposure but do not reliably distinguish between **active** and **latent TB**. [3] - Their sensitivity and specificity for diagnosing active TB, particularly in immunocompromised individuals, are generally **lower** than direct microbiological methods, making them unsuitable as a primary diagnostic tool for active disease.

Q: A patient with a history of heart valve replacement presents with fever and a new onset of a heart murmur. Blood cultures are pending. What is the most appropriate empiric antibiotic therapy?

Vancomycin and gentamicin. Vancomycin and gentamicin - This combination provides broad-spectrum coverage, targeting Gram-positive organisms (common in prosthetic valve endocarditis) with vancomycin and using gentamicin for synergistic activity [4] and coverage against potential gram-negative or difficult-to-treat organisms [3]. - The patient's history of heart valve replacement [2] and a new murmur strongly suggest infective endocarditis [5], requiring immediate empiric therapy covering common causative agents like staphylococci and streptococci [3]. [1] Penicillin and rifampin - Penicillin alone would not provide adequate coverage for MRSA or other resistant Gram-positive organisms commonly seen in prosthetic valve endocarditis [3]. - Rifampin is typically added later in the course of prosthetic valve endocarditis [2], often after initial de-escalation, due to its ability to penetrate biofilms, but it's not a first-line empiric choice due to rapid resistance development. Ceftriaxone and doxycycline - Ceftriaxone provides good coverage for streptococci [3] but lacks reliable activity against MRSA, a significant concern in prosthetic valve endocarditis. - Doxycycline has limited utility as a primary empiric agent for severe infective endocarditis, as its spectrum and bactericidal activity are not sufficient for this indication. Linezolid and azithromycin - Linezolid is effective against Gram-positive bacteria, including MRSA [3], but it is typically reserved for cases where vancomycin is contraindicated or ineffective, and its use as a primary empiric agent in this setting is less common. - Azithromycin has a narrow spectrum against common endocarditis pathogens and is generally not recommended for the treatment of severe bacterial endocarditis.

Q: A patient with a history of HIV presents with multiple ring-enhancing lesions in the brain, and serology is positive for IgG antibodies against Toxoplasma. What is the most likely diagnosis?

Toxoplasmosis. ***Toxoplasmosis*** - **Ring-enhancing lesions** in the brain of an **HIV patient** with **positive IgG antibodies against *Toxoplasma*** is highly characteristic of central nervous system toxoplasmosis [1]. - This opportunistic infection is common in individuals with **severe immunosuppression**, where dormant *Toxoplasma gondii* cysts reactivate [1]. *Cryptococcal meningitis* - While common in HIV patients, it typically presents with **meningeal symptoms** and diffuse enhancement or hydrocephalus on imaging, rather than distinct ring-enhancing lesions. - Diagnosis usually involves detecting **Cryptococcal antigen** in CSF or blood. *Cerebral malaria* - This is a rare complication of malaria, primarily seen in **endemic areas**, and is characterized by coma, seizures, and other neurological signs. - It does not typically present with **ring-enhancing lesions** and is not directly associated with HIV. *Cysticercosis* - Caused by the larval stage of **Taenia solium**, it can cause ring-enhancing lesions but is usually acquired through ingestion of contaminated food. - While possible, the combination of **HIV** and **positive *Toxoplasma* serology** makes toxoplasmosis a much more likely diagnosis in this clinical context.

Q: A traveler returning from a tropical country presents with fever and a single enlarging ulcer on the leg. Culture reveals gram-negative rods. Assess the diagnosis and consider the regional epidemiology.

Melioidosis. ***Melioidosis*** - **Melioidosis** is caused by the gram-negative bacterium *Burkholderia pseudomallei*, which is endemic in tropical regions, particularly Southeast Asia and Northern Australia. - The presentation of a **single enlarging ulcer** with accompanying **fever** in a traveler returning from a tropical country is highly consistent with localized invasive melioidosis [1]. *Buruli ulcer* - **Buruli ulcer** is caused by *Mycobacterium ulcerans* and is characterized by a painless, progressively destructive skin lesion, but it is not typically associated with fever in its early stages. - The causative agent is an acid-fast bacillus, not a gram-negative rod, which would differentiate it from the culture findings. *Cutaneous leishmaniasis* - **Cutaneous leishmaniasis** presents as skin lesions, often ulcers, after exposure in endemic areas, but it is caused by *Leishmania* parasites, not bacteria [1]. - Culture would reveal parasites, not gram-negative rods, and treatment would differ significantly. *Tularemia* - **Tularemia** can cause an ulceroglandular disease, but it is typically associated with exposure to infected animals (e.g., rabbits, rodents, ticks) in temperate regions like North America. - While *Francisella tularensis* is a gram-negative rod, the geographic context and the specific presentation of a single enlarging ulcer are less typical for tularemia compared to melioidosis.

Q: A patient with known rheumatoid arthritis presents with new-onset fever, fatigue, and a significant drop in hemoglobin. What is the most likely cause, and how should it be managed?

Infection; initiate broad-spectrum antibiotics. Infection; initiate broad-spectrum antibiotics - A patient with **rheumatoid arthritis (RA)** is commonly on **immunosuppressive therapy**, which increases the risk of infection [1]. - The triad of **fever**, **fatigue**, and new-onset **anemia** in an immunosuppressed patient strongly suggests an undiagnosed infection requiring prompt and aggressive treatment with **broad-spectrum antibiotics** [1]. *Rheumatoid vasculitis; increase immunosuppression* - While rheumatoid vasculitis can cause systemic symptoms and anemia, **fever** is not its primary feature, and it typically presents with other organ involvement such as **skin ulcers**, **neuropathy**, or **glomerulonephritis**. - Without clear evidence of vasculitis, simply increasing immunosuppression could worsen an underlying infection [1]. *Iron deficiency anemia; start iron supplements* - **Iron deficiency anemia** can cause fatigue and a drop in hemoglobin, but it does **not typically present with fever** as a primary symptom. - While RA patients can develop chronic disease anemia, new-onset significant anemia with fever is less likely to be solely due to iron deficiency [2]. *Gastrointestinal bleed; perform endoscopy* - A **gastrointestinal bleed** can certainly cause a drop in hemoglobin and fatigue, and GI complications are common in RA patients due to NSAID use [2]. - However, GI bleeding typically presents with **melena**, **hematochezia**, or **hemodynamic instability**, and **fever** is not a characteristic symptom of an uncomplicated GI bleed.

Q: A 35-year-old HIV-positive patient presents with a chronic cough and bloody sputum. A chest X-ray shows cavitary lesions. Which diagnostic test is the most appropriate?

Sputum culture for Mycobacterium. ***Sputum culture for Mycobacterium*** - Given the patient's **HIV-positive status**, **chronic cough**, **bloody sputum**, and **cavitary lesions** on chest X-ray, **tuberculosis** is highly suspected [1]. - A **sputum culture for Mycobacterium tuberculosis** is the definitive diagnostic test to confirm the presence of the bacteria and determine drug susceptibility [2]. *Blood culture for fungi* - While fungal infections can occur in HIV patients, **chronic cough**, **bloody sputum**, and **cavitary lesions** are not the primary or most characteristic presentation for common opportunistic fungal infections like PCP or cryptococcosis [1]. - **Microbiological sputum examination (e.g., fungal culture or staining)** might be considered if initial tests for TB are negative, but it is not the most appropriate initial diagnostic test given the strong suspicion for TB. *PCR for HIV viral load* - An **HIV viral load test** measures the amount of HIV in the blood and is used to monitor disease progression and response to antiretroviral therapy [3]. - It does **not directly diagnose the cause of the respiratory symptoms** and cavitary lesions in this patient. *CT scan of the chest* - A **CT scan of the chest** would provide more detailed anatomical information about the cavitary lesions and lung involvement. - However, it is an **imaging study** and does not provide a definitive microbiological diagnosis of the underlying infection.

Q: A patient presents with fever, eschar, and regional lymphadenopathy after hiking in an area known for mite infestation. What is the most likely diagnosis?

Scrub typhus. ***Scrub typhus*** - The classic triad of **fever**, an **eschar** at the bite site, and **regional lymphadenopathy** in an endemic area (often associated with mite exposure) is highly indicative of scrub typhus [1]. - Caused by **_Orientia tsutsugamushi_**, transmitted by the bite of infected **chiggers** (larval mites) [1]. *Rocky Mountain spotted fever* - Characterized by a **maculopapular rash** that typically begins on the wrists and ankles and spreads centrally, often involving palms and soles. - While it can present with fever and may be tick-borne, the presence of an **eschar is atypical**. *Typhus* - Includes **epidemic typhus** (lice-borne, caused by _Rickettsia prowazekii_) and **murine typhus** (flea-borne, caused by _Rickettsia typhi_) [2]. - Both forms primarily present with **fever and rash** (macular or maculopapular) but generally **do not feature an eschar** at the bite site [2]. *Lyme disease* - The hallmark is **erythema migrans**, a characteristic bull's-eye rash, along with fever, headache, and fatigue. - It is a **tick-borne illness** but an **eschar is not a typical presentation**, and the rash is distinct from that seen in scrub typhus.

Question 1

A 25-year-old woman presents with fever, rash, and joint pain after returning from Southeast Asia. What is the most likely diagnosis?

Accepted Answer

Dengue fever

Answer

Malaria

Answer

Typhoid fever

Answer

Leptospirosis

Question 2

A patient presents with fever, headache, and a bull's-eye rash. What is the most likely diagnosis, and which vector is associated with this disease?

Accepted Answer

Lyme disease, Ixodes tick

Answer

Rocky Mountain spotted fever, Dermacentor tick

Answer

Ehrlichiosis, Amblyomma tick

Answer

Babesiosis, Ixodes tick

Question 3

Which test is used to diagnose active tuberculosis?

Accepted Answer

Sputum culture

Answer

Mantoux test

Answer

Chest X-ray

Answer

ELISA

Question 4

A patient with a history of hematopoietic stem cell transplantation presents with fever and sinus pain. Imaging reveals sinusitis with possible fungal involvement. What is the most appropriate management?

Accepted Answer

Initiate amphotericin B and consider surgical debridement

Answer

Perform a biopsy and wait for histopathology results

Answer

Start empirical antibacterial therapy and monitor response

Answer

Administer corticosteroids to reduce inflammation

Question 5

A patient with a history of heart valve replacement presents with fever and a new onset of a heart murmur. Blood cultures are pending. What is the most appropriate empiric antibiotic therapy?

Accepted Answer

Vancomycin and gentamicin

Answer

Penicillin and rifampin

Answer

Ceftriaxone and doxycycline

Answer

Linezolid and azithromycin

Question 6

A patient with a history of HIV presents with multiple ring-enhancing lesions in the brain, and serology is positive for IgG antibodies against Toxoplasma. What is the most likely diagnosis?

Accepted Answer

Toxoplasmosis

Answer

Cryptococcal meningitis

Answer

Cerebral malaria

Answer

Cysticercosis

Question 7

A traveler returning from a tropical country presents with fever and a single enlarging ulcer on the leg. Culture reveals gram-negative rods. Assess the diagnosis and consider the regional epidemiology.

Accepted Answer

Melioidosis

Answer

Buruli ulcer

Answer

Cutaneous leishmaniasis

Answer

Tularemia

Question 8

A patient with known rheumatoid arthritis presents with new-onset fever, fatigue, and a significant drop in hemoglobin. What is the most likely cause, and how should it be managed?

Accepted Answer

Infection; initiate broad-spectrum antibiotics

Answer

Rheumatoid vasculitis; increase immunosuppression

Answer

Iron deficiency anemia; start iron supplements

Answer

Gastrointestinal bleed; perform endoscopy

Question 9

A 35-year-old HIV-positive patient presents with a chronic cough and bloody sputum. A chest X-ray shows cavitary lesions. Which diagnostic test is the most appropriate?

Accepted Answer

Sputum culture for Mycobacterium

Answer

Blood culture for fungi

Answer

PCR for HIV viral load

Answer

CT scan of the chest

Question 10

A patient presents with fever, eschar, and regional lymphadenopathy after hiking in an area known for mite infestation. What is the most likely diagnosis?

Accepted Answer

Scrub typhus

Answer

Rocky Mountain spotted fever

Answer

Typhus

Answer

Lyme disease

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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