Infectious Diseases — MCQs

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 121: Brill-Zinsser disease is a recrudescent form of which infection?

A. Recrudescence of R prowazekii infection (Correct Answer)
B. Recrudescence of R typhi infection
C. None of the options
D. Recrudescence of Coxiella burnetii infection

Explanation: **Recrudescence of R prowazekii infection** - Brill-Zinsser disease is a **late-onset complication** of **epidemic typhus**, caused by *Rickettsia prowazekii*. - It occurs years after the initial infection, due to **reactivation of dormant bacteria** in the body. *Recrudescence of R typhi infection* - *Rickettsia typhi* causes **murine typhus**, but its recrudescent form is not referred to as Brill-Zinsser disease. - Murine typhus is typically a **milder disease** compared to epidemic typhus. *Recrudescence of Coxiella burnetii infection* - *Coxiella burnetii* causes **Q fever**, which can have a chronic form but is not a recrudescence of a typhus infection. - Q fever presents with different clinical manifestations, such as **endocarditis** or **hepatitis**, not typically a rash or neurological symptoms seen in typhus. *None of the options* - This option is incorrect because the specific recrudescent form described in the question clearly points to *Rickettsia prowazekii*.

Question 122: Which of the following statements about Legionella is most accurate?

A. Prolonged carriers are common.
B. There is no human-to-human transmission.
C. Aerosol inhalation is a common mode of transmission. (Correct Answer)
D. All of the options are true

Explanation: ***Aerosol inhalation is a common mode of transmission.*** - **Legionella pneumophila** is primarily transmitted through **inhalation of aerosolized water droplets** containing the bacteria - Common sources include **cooling towers, air conditioning systems, hot tubs, showerheads**, and decorative fountains - This is the **most defining and clinically important characteristic** of Legionella transmission - Understanding aerosol transmission is crucial for **outbreak control and prevention strategies** *There is no human-to-human transmission.* - This statement is **medically accurate** - Legionella does NOT spread from person to person - However, this is a secondary characteristic compared to the primary transmission mode - The question asks for the **most accurate** statement, making aerosol transmission more definitive *Prolonged carriers are common.* - This is **INCORRECT** - Legionella does NOT cause a chronic carrier state in humans - The bacteria **colonizes environmental water systems and amoebae** (like *Acanthamoeba* and *Naegleria*), not human hosts - Humans are accidental hosts through aerosol exposure *All of the options are true* - This is **INCORRECT** because "Prolonged carriers are common" is false - Only two of the three substantive statements are true

Question 123: Patient came from Nagaland and shows positive test with OXK antigen. Diagnosis is?

A. Trench fever
B. Scrub typhus (Correct Answer)
C. Endemic typhus
D. Epidemic typhus

Explanation: ***Scrub typhus*** - A **positive Weil-Felix test** for the **OXK antigen** is highly suggestive of scrub typhus, particularly if the patient is from an endemic region like Nagaland. - Scrub typhus is caused by **_Orientia tsutsugamushi_**, transmitted by **chiggers** (larval mites), often presenting with fever and an **eschar**. *Trench fever* - Caused by **_Bartonella quintana_** and transmitted by the **human body louse**. - It does not typically show a positive Weil-Felix test for the OXK antigen. *Endemic typhus* - Also known as **murine typhus**, it is caused by **_Rickettsia typhi_** and transmitted by **fleas** (e.g., rat flea). - While it causes a positive Weil-Felix test, it usually involves the **OX19 antigen**, not OXK. *Epidemic typhus* - Caused by **_Rickettsia prowazekii_** and transmitted by the **human body louse**. - It yields a positive Weil-Felix test for the **OX19 and OX2 antigens**, but typically not for OXK.

Question 124: An 18-year-old girl in an endemic area presents with acute watery diarrhea. Considering the common infectious agents, what is the most likely causative agent?

A. Rotavirus
B. V. cholerae (Correct Answer)
C. Salmonella
D. Shigella

Explanation: ***V. cholerae*** - **Cholera** presents with **acute watery diarrhea**, often described as "rice water stools," which can lead to rapid dehydration. - The patient resides in an **endemic area**, significantly increasing the likelihood of *Vibrio cholerae* infection. *Rotavirus* - While *Rotavirus* causes acute watery diarrhea, it is more common in **infants and young children** (under 5 years old), making it less likely in an 18-year-old. - *Rotavirus* infections are typically seasonal in temperate climates and less tied to endemic area status in adults. *Salmonella* - *Salmonella* infections typically present with **bloody-mucoid diarrhea** (dysentery), fever, and abdominal cramps, rather than purely watery diarrhea. - While *Salmonella* can cause watery diarrhea, the classic presentation often involves more systemic symptoms and an inflammatory response. *Shigella* - *Shigella* infections also typically cause **dysentery**, characterized by **bloody diarrhea**, fever, and tenesmus, due to invasion of the intestinal mucosa. - The absence of blood in the stool and the description of "acute watery diarrhea" make *Shigella* less probable than *V. cholerae*.

Question 125: The most common causative organism for traveler's diarrhea is which of the following?

A. E. coli (Correct Answer)
B. Shigella
C. Norwalk virus
D. Rotavirus

Explanation: ***Correct: E. coli*** - **Enterotoxigenic Escherichia coli (ETEC)** is the most frequent cause of **traveler's diarrhea** worldwide, accounting for **30-40% of cases**. - It produces heat-labile (LT) and heat-stable (ST) enterotoxins that lead to **watery diarrhea** without significant fever or bloody stools. - Particularly common in travelers to **developing countries** in Latin America, Africa, and South Asia. *Incorrect: Shigella* - While it can cause severe diarrheal illness (dysentery) characterized by **bloody stools**, it accounts for only **5-10% of traveler's diarrhea cases**. - **Shigella infection** is more often associated with person-to-person spread and outbreaks in crowded settings. *Incorrect: Norwalk virus* - Also known as **Norovirus**, it is a very common cause of **gastroenteritis** globally, affecting people of all ages. - However, it is primarily responsible for widespread outbreaks, often in cruise ships or institutions, rather than being the predominant cause of individual traveler's diarrhea cases. *Incorrect: Rotavirus* - This virus is a major cause of **severe dehydrating diarrhea** in **infants and young children** worldwide. - While it can affect travelers, it is less common in adults and generally not considered the predominant cause of traveler's diarrhea in the general traveling population.

Question 126: Prusiner was awarded the Nobel Prize in 1997 for his work in:

A. Prion proteins (Correct Answer)
B. Gene therapy techniques
C. Neurodegenerative disorders
D. Insulin production methods

Explanation: ***Prion proteins*** - Dr. Stanley B. Prusiner received the Nobel Prize in Physiology or Medicine in 1997 for his discovery of **prions**—infectious protein particles responsible for transmissible spongiform encephalopathies. - His work revolutionized our understanding of how certain diseases can be transmitted by **misfolded proteins** without genetic material. *Gene therapy techniques* - Gene therapy involves introducing genetic material into cells to treat diseases and was developed through the work of multiple scientists over several decades. - While a significant advancement, this was not the specific area for which Prusiner was awarded the Nobel Prize. *Neurodegenerative disorders* - While prions cause **neurodegenerative disorders** (e.g., Creutzfeldt-Jakob disease), Prusiner's groundbreaking work was specifically on the nature of the infectious agent itself (prions), not the general category of neurodegenerative disorders. - Many scientists contribute to understanding neurodegenerative diseases, but Prusiner's unique contribution was identifying a novel infectious mechanism. *Insulin production methods* - Insulin was first isolated and identified by Banting and Macleod in the early 20th century, leading to a Nobel Prize in 1923. - Modern methods for insulin production, including recombinant DNA technology, were developed by other researchers and companies at different times.

Question 127: Among the following E. coli variants, which is most commonly associated with persistent diarrhea in adults?

A. Enteroaggregative Escherichia coli (EAEC) (Correct Answer)
B. Enteropathogenic Escherichia coli (EPEC)
C. Enterotoxigenic Escherichia coli (ETEC)
D. Enteroinvasive Escherichia coli (EIEC)

Explanation: ***Enteroaggregative Escherichia coli (EAEC)*** - EAEC is characterized by its distinctive **aggregative adherence** to intestinal cells, leading to chronic inflammation and persistent diarrhea, particularly in adults and immunocompromised individuals. - It produces **enteroaggregative heat-stable toxin (EAST1)** and **pet-like cytotoxins** that contribute to fluid secretion and mucosal damage. *Enteropathogenic Escherichia coli (EPEC)* - EPEC is a major cause of **infant diarrhea**, especially in developing countries, but is generally not associated with persistent diarrhea in adults. - It forms characteristic **attaching and effacing (A/E) lesions** on intestinal epithelial cells, destroying microvilli and impairing absorption. *Enterotoxigenic Escherichia coli (ETEC)* - ETEC is the most common cause of **traveler's diarrhea** and primarily causes acute, watery diarrhea due to the production of **heat-labile (LT)** and/or **heat-stable (ST) toxins**. - While it can cause significant dehydrating diarrhea, it is typically self-limiting and rarely leads to persistent symptoms in adults. *Enteroinvasive Escherichia coli (EIEC)* - EIEC causes a disease similar to **shigellosis**, characterized by **fever, cramps, and dysentery** (bloody, mucoid stools), by invading and destroying intestinal epithelial cells. - It is not commonly associated with persistent diarrhea but rather with an acute, inflammatory colonic infection.

Question 128: Which organism does not require a vector for transmission?

A. Rickettsia rickettsii
B. Borrelia recurrentis
C. Rickettsia prowazekii
D. Coxiella burnetii (Correct Answer)

Explanation: ***Coxiella burnetii*** - This organism causes **Q fever** and does not require an arthropod vector; it is primarily transmitted via **aerosols** from infected animals. - Humans usually acquire the infection by inhaling **contaminated aerosols** from infected livestock (cattle, sheep, goats). *Rickettsia prowazekii* - This bacterium is the causative agent of **epidemic typhus** and is transmitted by the **human body louse** (*Pediculus humanus corporis*). - The louse acts as a **biological vector**, acquiring the organism from an infected human and transmitting it through its feces, which are then scratched into the skin. *Rickettsia rickettsii* - This organism causes **Rocky Mountain Spotted Fever (RMSF)** and is transmitted by **ticks**, primarily the American dog tick (*Dermacentor variabilis*) and the Rocky Mountain wood tick (*Dermacentor andersoni*). - Ticks serve as both **vectors** and **reservoirs** for *Rickettsia rickettsii*. *Borrelia recurrentis* - This spirochete causes **louse-borne relapsing fever** and is transmitted by the **human body louse** (*Pediculus humanus corporis*). - Transmission occurs when the louse is crushed and its hemolymph, containing the bacteria, comes into contact with broken skin or mucous membranes.

Question 129: Which of the following viruses is not spread by the fecal-oral route?

A. Rotavirus
B. Hepatitis B virus (HBV) (Correct Answer)
C. Hepatitis E virus (HEV)
D. Norwalk virus (Norovirus)

Explanation: ***Hepatitis B virus (HBV)*** - HBV is primarily transmitted through **percutaneous** or **mucosal exposure** to infectious blood or body fluids, such as during sex, sharing needles, or from mother to child. - It is not typically spread via the **fecal-oral route**. *Hepatitis E virus (HEV)* - HEV is a common cause of **acute viral hepatitis** and is predominantly transmitted through the **fecal-oral route**, often via contaminated drinking water. - High viral loads are shed in the feces of infected individuals, leading to environmental contamination. *Rotavirus* - Rotavirus is a major cause of **severe diarrheal disease** in infants and young children, and its transmission occurs almost exclusively through the **fecal-oral route**. - The virus is highly stable in the environment and can spread rapidly in settings with poor hygiene. *Norwalk virus (Norovirus)* - Norovirus is a highly contagious virus that causes **gastroenteritis** and is notorious for outbreaks in crowded settings; its primary mode of transmission is the **fecal-oral route**. - It can also spread through contaminated food and water, or direct contact with infected individuals.

Question 130: Which of the following conditions is most commonly classified as a sexually transmitted disease?

A. Trichomoniasis
B. Giardiasis
C. Amoebiasis
D. Chlamydia (Correct Answer)

Explanation: ***Chlamydia*** - **Chlamydia trachomatis** is a bacterial infection that is exclusively transmitted through **sexual contact**, making it a classic sexually transmitted disease. - It often presents with **asymptomatic infections** but can lead to serious complications if untreated, such as pelvic inflammatory disease (PID) and infertility. *Trichomoniasis* - Caused by the **protozoan Trichomonas vaginalis**, it is also a sexually transmitted infection. - While sexually transmitted, **Chlamydia** is generally more prevalent and often considered the *most commonly* classified or reported bacterial STD. *Giardiasis* - Caused by the parasite **Giardia lamblia**, it is typically acquired through the ingestion of contaminated water or food. - It is primarily an **enteric infection** and not classified as a sexually transmitted disease, although anal-oral contact can be a route of transmission in some cases. *Amoebiasis* - Caused by the parasite **Entamoeba histolytica**, it is spread through the fecal-oral route, usually via contaminated food or water. - **Amoebiasis** is an infection of the intestines and liver, and it is not considered a sexually transmitted disease.

Practice by Chapter

Respiratory Tract Infections

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Urinary Tract Infections

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Gastrointestinal Infections

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Skin and Soft Tissue Infections

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Central Nervous System Infections

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Bone and Joint Infections

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Cardiovascular Infections

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Sexually Transmitted Infections

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Zoonotic Infections

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Bloodstream Infections and Sepsis

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Fever of Unknown Origin

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Infections in Immunocompromised Host

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