Infectious Diseases Practice Questions

Q: What is the epidemiological marker of Hepatitis B?

HBsAg. ### Explanation **Hepatitis B Surface Antigen (HBsAg)** is the correct answer because it is the first serological marker to appear in the blood after infection (even before symptoms or biochemical changes). In epidemiological studies, HBsAg is used to determine the **prevalence** of HBV in a population and to identify both acute and chronic carriers. Its presence for more than 6 months defines a chronic carrier state. **Analysis of Incorrect Options:** * **Anti-HBs (HBsAb):** This antibody indicates **immunity** to HBV, either through successful vaccination or recovery from a natural infection. It is not used as a marker for active disease prevalence. * **Anti-HBc (HBcAb):** This is a marker of **exposure**. IgM anti-HBc indicates recent/acute infection (and is the only marker positive during the "window period"), while IgG anti-HBc indicates past exposure. * **HBeAg:** This is a marker of **active viral replication and high infectivity**. While clinically significant for assessing the risk of transmission, it is not the primary epidemiological tool for screening populations. **High-Yield Clinical Pearls for NEET-PG:** * **Window Period:** The interval when HBsAg disappears but Anti-HBs has not yet appeared. **IgM anti-HBc** is the diagnostic marker here. * **Screening:** HBsAg is the standard marker for screening blood donors. * **Vaccine Response:** A person vaccinated against HBV will be **Anti-HBs positive** but **Anti-HBc negative** (since the vaccine contains only the surface antigen). * **Infectivity:** HBeAg and HBV-DNA levels are the best indicators of a patient's level of infectivity.

Q: Which of the following is true about the epidemiology of malaria?

The mosquito acts as the definitive host.. **Explanation:** In parasitology, the **definitive host** is defined as the host in which the parasite undergoes its **sexual cycle** of reproduction. In the case of Malaria (*Plasmodium* species), the sexual phase (sporogony) occurs within the female *Anopheles* mosquito. Conversely, the asexual phase (schizogony) occurs in humans, making **Man the intermediate host**. **Analysis of Options:** * **Option D (Correct):** As stated above, the mosquito is the definitive host because the fusion of gametes (syngamy) and the formation of the oocyst occur in the mosquito's gut. * **Option A (Incorrect):** The **extrinsic incubation period** (the time taken for the parasite to develop inside the mosquito) is typically **10–21 days**, depending on the species and ambient temperature. It is rarely as low as 0 days. * **Option B (Incorrect):** In India, malaria transmission is seasonal and peaks during and after the **monsoon (July to November)** due to the availability of stagnant water for mosquito breeding. January to June is generally the dry/off-season. * **Option C (Incorrect):** Man is the intermediate host because only asexual reproduction occurs in human hepatocytes and erythrocytes. **High-Yield NEET-PG Pearls:** * **Intrinsic Incubation Period:** The time interval between the bite of an infected mosquito and the onset of clinical symptoms in man (usually 10–14 days for *P. falciparum*). * **Infective Stage:** For humans, it is the **Sporozoite**; for mosquitoes, it is the **Gametocyte**. * **Temperature:** Optimal transmission occurs between **20°C and 30°C**. Transmission ceases if the temperature falls below 16°C or rises above 30°C. * **Humidity:** Relative humidity of **60% or more** is essential for the survival of the mosquito.

Q: Which disease is caused by a mite?

Scrub typhus. **Explanation:** **Scrub Typhus (Correct Answer):** Scrub typhus is caused by the bacterium *Orientia tsutsugamushi*. It is transmitted to humans through the bite of the infected **larval stage of trombiculid mites** (commonly known as **chiggers** or harvest mites). These mites serve as both the vector and the reservoir (via transovarial transmission). A characteristic clinical feature is the **eschar**—a painless, black crusty lesion at the site of the mite bite. **Analysis of Incorrect Options:** * **B. Q Fever:** Caused by *Coxiella burnetii*. It is primarily an airborne zoonosis transmitted via inhalation of contaminated dust or aerosols from infected livestock (cattle, sheep, goats). It is not mite-borne. * **C. Trench Fever:** Caused by *Bartonella quintana*. It is transmitted by the **human body louse** (*Pediculus humanus corporis*). * **D. Endemic Typhus:** Also known as Murine typhus, it is caused by *Rickettsia typhi*. The primary vector is the **rat flea** (*Xenopsylla cheopis*). **High-Yield NEET-PG Pearls:** * **Vector Mnemonic:** Remember **"Mite-y Scrub"** to link Mites with Scrub Typhus. * **Drug of Choice:** Doxycycline is the gold standard treatment for all rickettsial diseases, including scrub typhus. * **Diagnostic Test:** The **Weil-Felix test** shows heterophile agglutination with **OX-K** antigens for Scrub Typhus (unlike Epidemic/Endemic typhus which react with OX-19 and OX-2). * **Habitat:** It is prevalent in areas with secondary scrub vegetation, often referred to as "tsutsugamushi triangles."

Q: What is the best method to identify cases of tuberculosis within a community?

Sputum smear examination. **Explanation:** In the context of public health and community medicine, **Sputum Smear Examination** (specifically for Acid-Fast Bacilli via Ziehl-Neelsen staining) remains the gold standard for **case identification** in a community setting. This is because it identifies "infectious cases"—those individuals who are actively shedding the bacteria and are responsible for the continued transmission of tuberculosis within the community. It is cost-effective, rapid, and highly specific for identifying the most contagious patients. **Analysis of Options:** * **A. Chest X-ray:** While highly sensitive, it is not specific for TB. It cannot differentiate between active infection, healed lesions, or other lung pathologies (like pneumonia or malignancy). It is used as a screening tool, not for definitive case identification. * **B. Mantoux Test:** This is a test of "infection," not "active disease." A positive result only indicates that the person has been exposed to *M. tuberculosis* at some point; it does not mean they currently have TB or are infectious. * **D. Sputum Culture:** This is the "absolute gold standard" for diagnosis and is more sensitive than a smear. However, it is **not the best method for community identification** because it takes 2–8 weeks to yield results, requires sophisticated laboratory infrastructure, and is expensive. **NEET-PG High-Yield Pearls:** * **Primary Tool:** Under the National TB Elimination Program (NTEP), **Sputum Smear** is the primary tool for diagnosis, though **NAAT (CBNAAT/Truenat)** is now the preferred initial diagnostic test where available. * **Infectivity:** One "smear-positive" case can infect 10–15 close contacts in a year if left untreated. * **Screening vs. Diagnosis:** In a community survey, Chest X-ray is the best **screening** tool, but Sputum examination is the best **diagnostic** tool.

Q: During an epidemic of hepatitis, fulminant hepatic failure is seen in:

Pregnant female. **Explanation:** The correct answer is **Pregnant female**. This question refers to an epidemic of **Hepatitis E Virus (HEV)**, which is the most common cause of water-borne epidemics of viral hepatitis in developing countries like India. **Why Pregnant Females?** While Hepatitis E is generally a self-limiting disease in the general population (case fatality rate <1%), it is notoriously severe in pregnant women, particularly during the **third trimester**. In this group, the case fatality rate can soar to **15–25%** due to **Fulminant Hepatic Failure (FHF)** and associated complications like Disseminated Intravascular Coagulation (DIC). The exact pathogenesis is linked to hormonal changes and altered immune responses during pregnancy that promote viral replication and liver injury. **Analysis of Incorrect Options:** * **A & D (Malnourished child / Child <15 years):** In children, Hepatitis E is often asymptomatic or results in a very mild, anicteric illness. It rarely leads to fulminant failure. * **C (Elderly individual):** While the elderly may have more comorbidities, they do not show the specific, disproportionately high mortality rate seen in pregnant women during HEV outbreaks. **High-Yield Clinical Pearls for NEET-PG:** * **Route of Transmission:** Fecal-oral (contaminated water). * **Incubation Period:** 2–9 weeks (Average: 6 weeks). * **Epidemiology:** HEV is the leading cause of **sporadic** and **epidemic** viral hepatitis in India. * **Zoonosis:** HEV Genotype 3 and 4 are associated with pig reservoirs (relevant for sporadic cases). * **Rule of Thumb:** If a question mentions "Epidemic Hepatitis" + "High mortality in pregnancy," the answer is always **Hepatitis E**.

Q: All are true regarding tetanus, except?

More common in winters and dry weather. **Explanation:** Tetanus is caused by the neurotoxin produced by *Clostridium tetani*, an anaerobic, Gram-positive, spore-forming bacillus [2]. **Why Option B is the correct answer (The Exception):** Tetanus does not show a preference for winter or dry weather. In fact, it is **more common during the rainy season and summer months**. This is because moisture favors the survival of spores in the soil, and agricultural activities (which increase the risk of injury) are more frequent during these periods [1]. **Analysis of other options:** * **Option A:** Transmission occurs when spores of *C. tetani* are introduced into the body through contaminated wounds, lacerations, or even minor pricks [2]. Anaerobic conditions (necrotic tissue) allow spores to germinate [5]. * **Option C:** The primary reservoir is the **soil**, where spores can persist for years. The organism is also a commensal in the **intestines of humans and animals** (especially horses), which further contaminates the soil through feces [2], [4]. * **Option D:** Tetanus is a unique infectious disease because it offers **no herd immunity** (it is not person-to-person transmissible) and **no lifelong immunity** after a natural infection. The amount of tetanospasmin required to cause disease is so small that it is sub-immunogenic (not enough to trigger an immune response) [2]. **NEET-PG High-Yield Pearls:** * **Tetanospasmin:** A potent neurotoxin that blocks the release of inhibitory neurotransmitters (**GABA and Glycine**) at the motor nerve endings, leading to spastic paralysis [5]. * **Clinical Signs:** Risus sardonicus (grimace), Trismus (lockjaw), and Opisthotonus (arch-like body posture) [1]. * **Prevention:** Active immunization with Tetanus Toxoid is the only reliable way to prevent the disease [3].

Q: Severity of plague spread is detected by which index?

Cheopis index. **Explanation** Plague is a zoonotic disease caused by *Yersinia pestis*, primarily transmitted to humans by the bite of infected rat fleas. To monitor the risk of an outbreak, epidemiologists use specific **Flea Indices**. **1. Why Cheopis Index is correct:** The **Cheopis Index** (or Specific Flea Index) measures the average number of *Xenopsylla cheopis* (the oriental rat flea) per rat. It is the most critical indicator because *X. cheopis* is the most efficient vector for transmitting plague. A **Cheopis Index greater than 1** is considered a high-risk threshold, indicating that the flea population is sufficient to sustain a plague epidemic in the community. **2. Analysis of Incorrect Options:** * **Total Flea Index:** This represents the average number of fleas of all species found per rat. While it indicates general infestation, it is less specific than the Cheopis index because not all flea species are equally effective at transmitting plague. * **Burrow’s Index:** This measures the average number of fleas found within a rat's burrow rather than on the host itself. It is used to study flea ecology but is not the standard for predicting epidemic severity. * **Chandler’s Index:** This is used in the epidemiology of **Hookworm infestation**. It measures the average number of hookworm eggs per gram of stool, used to estimate the "worm burden" in a community. **High-Yield Facts for NEET-PG:** * **Vector:** *Xenopsylla cheopis* (most common/efficient); *X. astia* and *X. braziliensis* are others. * **Critical Threshold:** A Cheopis Index **> 1** indicates an explosive outbreak risk. * **Percentage Flea Index:** The percentage of rats infested with any flea species (a value > 30% is significant). * **Blocking Phenomenon:** *Y. pestis* multiplies in the flea's proventriculus, causing a "block" that makes the flea hungry and more likely to bite humans repeatedly.

Q: Which of the following is NOT an anthropozoonosis?

Dracunculiasis. ### Explanation The core of this question lies in the definition of **Anthropozoonosis**. Anthropozoonoses are infectious diseases that are naturally transmitted from vertebrate animals to humans. In these cycles, humans are usually "accidental" hosts, and the disease can be maintained in nature without human intervention. **Why Dracunculiasis is the correct answer:** **Dracunculiasis (Guinea worm disease)** is considered an **anthropozoonosis** (or more specifically, a human-specific infection). The life cycle involves humans and a crustacean intermediate host (*Cyclops*), but it lacks a vertebrate animal reservoir. Because there is no vertebrate animal involved in the natural transmission cycle to humans, it does not meet the definition of a zoonosis. **Analysis of Incorrect Options:** * **Rabies:** A classic viral anthropozoonosis transmitted to humans via the bite of infected vertebrate animals (dogs, bats, monkeys). * **Plague:** A bacterial zoonosis caused by *Yersinia pestis*. The natural reservoir is wild rodents (vertebrates), with transmission to humans occurring via flea bites. * **Hydatid Cyst:** Caused by *Echinococcus granulosus*. It is a parasitic zoonosis where dogs are the definitive hosts and sheep/cattle are intermediate hosts. Humans are accidental "dead-end" hosts. **NEET-PG High-Yield Pearls:** * **Definitions:** * *Anthropozoonosis:* Animal $\rightarrow$ Man (e.g., Rabies, Plague, Anthrax). * *Zooanthroponosis:* Man $\rightarrow$ Animal (e.g., Human Tuberculosis to cattle). * *Amphixenosis:* Transmission in both directions (e.g., Salmonellosis). * **Eradication Status:** Dracunculiasis is on the verge of global eradication. India was certified Guinea worm-free by the WHO in February 2000. * **Vector:** The intermediate host for *Dracunculus medinensis* is the **Cyclops** (Water flea). Filtering water is the primary preventive measure.

Question 1

What is the epidemiological marker of Hepatitis B?

Accepted Answer

HBsAg

Answer

Anti-HBs

Answer

Anti-HBc

Answer

HBeAg

Question 2

Which of the following is true about the epidemiology of malaria?

Accepted Answer

The mosquito acts as the definitive host.

Answer

The extrinsic incubation period is 0-14 days.

Answer

In India, it is common during January to June.

Answer

Man acts as the definitive host.

Question 3

Which disease is caused by a mite?

Accepted Answer

Scrub typhus

Answer

Fever

Answer

Trench fever

Answer

Endemic typhus

Question 4

What is the best method to identify cases of tuberculosis within a community?

Accepted Answer

Sputum smear examination

Answer

Chest X-ray

Answer

Mantoux test

Answer

Sputum culture and sensitivity

Question 5

During an epidemic of hepatitis, fulminant hepatic failure is seen in:

Accepted Answer

Pregnant female

Answer

Malnourished child

Answer

Elderly individual

Answer

Child less than 15 years of age

Question 6

Which of the following is a true statement regarding the tuberculin test?

Accepted Answer

An induration of more than 10 mm after 72 hours indicates a positive test

Answer

Used for the definitive diagnosis of active tuberculosis

Answer

Measures the incidence of tuberculosis infection in a population

Answer

Measures the level of acquired immunity against tuberculosis

Question 7

All are true regarding tetanus, except?

Accepted Answer

More common in winters and dry weather

Answer

Transmission through contaminated wounds and injuries

Answer

Reservoir is soil and intestines of humans and animals

Answer

No herd immunity or lifelong immunity

Question 8

Severity of plague spread is detected by which index?

Accepted Answer

Cheopis index

Answer

Total Flea index

Answer

Burrow's index

Answer

Chandler's index

Question 9

Which of the following is NOT an anthropozoonosis?

Accepted Answer

Dracunculiasis

Answer

Rabies

Answer

Plague

Answer

Hydatid cyst

Question 10

Which of the following statements regarding the Mantoux test is NOT true?

Accepted Answer

A positive test does not definitively mean the person has active TB disease.

Answer

The test is read before 48 hours.

Answer

6-9 mm induration suggests a high likelihood of developing TB.

Answer

New TB cases are less likely to occur in tuberculin-negative individuals compared to tuberculin reactors.

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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