Infectious Diseases Practice Questions

Q: Which of the following is NOT true about diphtheria?

Chemoprophylaxis is done with rifampicin.. **Explanation:** **Why Option B is the correct answer (False statement):** The drug of choice for chemoprophylaxis in diphtheria is **Erythromycin** (oral) or a single dose of **Benzathine Penicillin G** (IM). Rifampicin is primarily used for chemoprophylaxis in *Meningococcal meningitis* and *H. influenzae* type b infections, not diphtheria. **Analysis of other options:** * **Option A:** The incubation period for *Corynebacterium diphtheriae* is typically short, ranging from **2 to 5 days**, though it can occasionally extend to 10 days. * **Option C:** Management of close contacts is a high-yield topic. Asymptomatic household contacts who were previously immunized should receive a **booster dose** of a diphtheria toxoid-containing vaccine to ensure immediate protective immunity. * **Option D:** Diphtheria Antitoxin (DAT) is the cornerstone of treatment. It must be administered **immediately** based on clinical suspicion to neutralize circulating toxins before they bind to tissues (myocardium and nerves), as it cannot neutralize toxin already fixed to cells. **High-Yield Clinical Pearls for NEET-PG:** * **Schick Test:** Used to demonstrate immunity/susceptibility to diphtheria. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Morphology:** Gram-positive bacilli with "Chinese letter" or cuneiform arrangement; contains **Volutin/Metachromatic granules** (Albert’s stain). * **Toxin:** It is an A-B toxin that inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Q: Which of the following diseases is NOT transmitted by Aedes aegypti?

Filariasis. **Explanation:** The correct answer is **Filariasis**. While *Aedes aegypti* is a prolific vector for several viral diseases, it is not the primary vector for Lymphatic Filariasis in India. **1. Why Filariasis is the correct answer:** In India and most endemic regions, **Lymphatic Filariasis** (caused by *Wuchereria bancrofti*) is primarily transmitted by the **Culex quinquefasciatus** mosquito. *Culex* breeds in dirty, stagnant water (sewage/sullage), whereas *Aedes* prefers clean, man-made containers. While some species of *Aedes* (like *Aedes polynesiensis*) transmit filariasis in Pacific islands, it is not the vector for the disease in the context of standard medical exams unless specified. **2. Analysis of incorrect options:** * **Yellow Fever:** *Aedes aegypti* is the principal vector. Although not present in India, it remains a high-yield topic due to strict quarantine regulations. * **Dengue Fever:** *Aedes aegypti* (Tiger mosquito) is the primary vector, breeding in artificial collections of clean water (coolers, tires, flower pots). * **Chikungunya:** This is also transmitted by *Aedes aegypti* and *Aedes albopictus*. It often presents with severe arthralgia. **High-Yield Clinical Pearls for NEET-PG:** * **Aedes aegypti characteristics:** Known as the "Day Biter" (mostly early morning/late afternoon). It is a "nervous feeder" (bites multiple people to complete one meal) and has a short flight range (<100 meters). * **Zika Virus:** Also transmitted by *Aedes aegypti*. * **Vector Control:** The most effective method for *Aedes* control is **Environmental Modification** (source reduction) and using larvicides like **Abate (Temephos)**.

Q: What is the vector for Yellow fever?

Aedes. **Explanation:** **Yellow Fever** is a viral hemorrhagic disease caused by a **Flavivirus**. The correct answer is **Aedes** because the primary vector for transmitting the virus to humans is the **Aedes aegypti** mosquito (urban cycle). In the jungle cycle, other species like *Haemagogus* and *Sabethes* are involved. **Analysis of Options:** * **Aedes (Correct):** Specifically *Aedes aegypti*, which breeds in artificial containers and is a day-biter. It is also the vector for Dengue, Chikungunya, and Zika. * **Culex:** This mosquito is the primary vector for **Japanese Encephalitis**, West Nile Virus, and **Bancroftian Filariasis**. It typically breeds in dirty, stagnant water. * **Anopheles:** This is the well-known vector for **Malaria**. It breeds in clean, moving, or stagnant water. * **Mansonoides:** This mosquito is the vector for **Brugian Filariasis** (Malayan filariasis). It is unique because its larvae attach to the roots of aquatic plants like *Pistia*. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** 3 to 6 days. * **Vaccine:** The **17D vaccine** is a live attenuated vaccine. It provides immunity for life (as per WHO 2014 guidelines), though international travel regulations may still reference a 10-year validity. * **Contraindications:** The vaccine is contraindicated in infants <6 months, pregnant women (unless during an outbreak), and immunocompromised individuals (e.g., symptomatic HIV, thymus disorders). * **India Status:** India is "Yellow Fever receptive" (the vector is present, but the disease is not), which is why strict quarantine and vaccination certificates are required for travelers from endemic zones.

Q: Which of the following diseases is spread by mosquitoes?

Dengue. **Explanation:** **Dengue** is the correct answer because it is a viral infection transmitted to humans through the bite of infected female mosquitoes, primarily the **_Aedes aegypti_** species (and to a lesser extent, *Aedes albopictus*). These mosquitoes are "day-biters" and typically breed in stagnant clean water in urban settings. **Analysis of Incorrect Options:** * **Kala-azar (Visceral Leishmaniasis):** This is caused by the protozoan parasite *Leishmania donovani* and is transmitted by the bite of the infected female **Sandfly** (*Phlebotomus argentipes*). * **Trypanosomiasis:** African Trypanosomiasis (Sleeping Sickness) is transmitted by the **Tsetse fly** (*Glossina* species). American Trypanosomiasis (Chagas disease) is transmitted by **Triatomine bugs** (kissing bugs). * **Listeriosis:** This is a bacterial infection caused by *Listeria monocytogenes*. It is primarily a **foodborne illness** contracted by consuming contaminated dairy products (unpasteurized cheese), deli meats, or soil-contaminated vegetables. **High-Yield Clinical Pearls for NEET-PG:** * **Aedes aegypti** is also the vector for Yellow Fever, Zika virus, and Chikungunya. * **Extrinsic Incubation Period:** In Dengue, the virus requires 8–12 days of replication inside the mosquito before it can be transmitted to another human. * **Dengue Triad:** Fever, rash, and severe headache (retro-orbital pain). It is often called "Breakbone fever" due to intense joint and muscle pain. * **Key Diagnostic:** NS1 antigen is the marker of choice in the first 5 days; IgM antibodies appear after day 5.

Q: Which vector is most dangerous for transmitting plague?

Partially blocked flea. In the transmission of Plague (*Yersinia pestis*), the vector efficiency of the rat flea (*Xenopsylla cheopis*) depends on the phenomenon of **"blocking."** ### **Why "Partially Blocked Flea" is the Correct Answer** When a flea ingests blood containing *Y. pestis*, the bacteria multiply in the **proventriculus** (the flea's foregut), forming a biofilm. * In a **partially blocked flea**, the obstruction is incomplete. This allows the flea to survive longer than a fully blocked flea while still being able to transmit the bacteria. * Because the passage is narrowed, the flea struggles to suck blood, leading to "frantic biting" behavior. During these repeated attempts, contaminated blood is regurgitated back into the host's wound. * The combination of **prolonged survival** and **increased biting frequency** makes it the most dangerous and efficient vector. ### **Analysis of Incorrect Options** * **A. Blocked Flea:** While a fully blocked flea is highly infectious, the complete obstruction prevents any blood from entering the stomach. This leads to rapid dehydration and death of the flea (usually within 3-4 days), limiting its window of transmission. * **D. Unblocked Flea:** In an unblocked flea, the bacteria pass freely into the midgut. While the flea remains healthy, it does not experience the "hunger-driven" biting urge, and the mechanism of regurgitation is less efficient. ### **High-Yield Clinical Pearls for NEET-PG** * **Vector:** *Xenopsylla cheopis* (Oriental rat flea) is the most common vector. * **Bacillus:** *Yersinia pestis* (Gram-negative, safety-pin appearance/bipolar staining). * **Flea Index:** A "General Flea Index" **>1** is considered a danger signal for an impending plague outbreak. * **Temperature:** Blocking occurs more efficiently at temperatures below **27°C**; higher temperatures can "unblock" fleas, often ending an epidemic.

Q: Which of the following is NOT a zoonotic disease?

Giardiasis. ### Explanation **1. Why Giardiasis is the Correct Answer:** Giardiasis, caused by the protozoan parasite *Giardia duodenalis* (also known as *G. intestinalis* or *G. lamblia*), is primarily an **anthroponotic** disease. While some animal strains can occasionally infect humans, the vast majority of human infections occur through the **fecal-oral route** via contaminated water or person-to-person contact. It is not classified as a classic zoonosis because the primary reservoir and source of infection for humans are other humans. **2. Why the Other Options are Incorrect:** * **Leptospirosis:** A classic **direct zoonosis** caused by *Leptospira interrogans*. It is transmitted to humans through contact with water or soil contaminated by the urine of infected animals (primarily rodents). * **Brucellosis:** A major **zoonotic infection** (undulant fever) transmitted to humans from cattle, goats, or sheep through direct contact or the consumption of unpasteurized dairy products. * **Rabies:** A fatal **viral zoonosis** transmitted through the bite or scratch of an infected animal (most commonly dogs in India). **3. NEET-PG High-Yield Clinical Pearls:** * **Definition of Zoonoses:** Diseases and infections which are naturally transmitted between vertebrate animals and man (WHO). * **Giardiasis Key Fact:** It is the most common intestinal protozoan pathogen worldwide. In the small intestine, it causes malabsorption, leading to **steatorrhea** (foul-smelling, fatty stools). * **Leptospirosis:** Often presents as **Weil’s Disease** (triad of jaundice, renal failure, and hemorrhage). * **Brucellosis:** Occupational hazard for veterinarians, butchers, and dairy farmers. The standard diagnostic test is the **Standard Agglutination Test (SAT)**.

Q: The Aedes aegypti index should be less than what percentage?

1%. **Explanation:** The **Aedes aegypti index** (also known as the House Index) is a key entomological indicator used to monitor the risk of transmission for diseases like Dengue, Chikungunya, and Zika. It is defined as the percentage of houses found positive for Aedes aegypti larvae or pupae. **1. Why 1% is the correct answer:** According to World Health Organization (WHO) and National Vector Borne Disease Control Programme (NVBDCP) guidelines, an Aedes aegypti index of **less than 1%** is considered the "safety threshold." When the index is maintained below 1%, the risk of an epidemic outbreak is considered negligible. If the index rises above this level, it indicates a potential for disease transmission. **2. Analysis of Incorrect Options:** * **B (5%):** While a 5% threshold is sometimes used for the *Breteau Index* (number of positive containers per 100 houses) to indicate low risk, it is not the safety standard for the House Index. * **C & D (8% and 10%):** These values represent high-risk scenarios. An index exceeding 10% is often associated with a high probability of an explosive outbreak during the transmission season. **3. NEET-PG High-Yield Pearls:** * **House Index (HI):** (Houses positive / Houses inspected) × 100. * **Container Index (CI):** (Containers positive / Containers inspected) × 100. * **Breteau Index (BI):** (Number of positive containers / Total houses inspected) × 100. (Considered the best predictor of Aedes density). * **Aedes aegypti** is a "day biter," breeds in clean stagnant water (artificial containers), and exhibits "intermittent feeding" (biting multiple people to complete one blood meal), which increases its epidemic potential.

Q: A malarial survey is conducted in 50 villages having a population of one lakh. Out of 20,000 slides examined, 500 turned out to be malaria positive. What is the annual parasite incidence?

0.50%. ### Educational Explanation **Understanding Annual Parasite Incidence (API)** The Annual Parasite Incidence (API) is a crucial epidemiological index used under the National Vector Borne Disease Control Programme (NVBDCP) to measure the incidence of malaria in a community. It represents the number of confirmed malaria cases per 1,000 population per year. **Calculation:** The formula for API is: $$\text{API} = \frac{\text{Total number of positive slides in a year}}{\text{Total population}} \times 1000$$ Given in the question: * Total Population = 1,00,000 (One Lakh) * Total Positive Slides = 500 * (Note: The number of slides examined is used for the Annual Blood Examination Rate, not API). $$\text{API} = \frac{500}{1,00,000} \times 1000 = 5$$ An API of 5 per 1,000 population is equivalent to **0.5%** (since $5/1000 = 0.005$, and $0.005 \times 100 = 0.5\%$). Therefore, **Option C** is the correct answer. **Analysis of Incorrect Options:** * **Option A (20%):** This is mathematically incorrect and does not correlate with any standard malaria metric. * **Option B (5%):** This is a calculation error where the "per 1000" factor was likely confused with "per 100." * **Option D (0.40%):** This value is obtained if one incorrectly uses the number of slides examined (20,000) as the denominator instead of the total population. **High-Yield Clinical Pearls for NEET-PG:** * **API 10%**. * **Slide Positivity Rate (SPR):** (Total Positives / Total Slides Examined) × 100. In this case, SPR would be 2.5%. * **Slide Falciparum Rate (SFR):** (Total *P. falciparum* Positives / Total Slides Examined) × 100.

Q: Insecticide-treated bed nets (ITBNs) are typically treated with which class of insecticides?

Pyrethroids (e.g., Deltamethrin, Cyfluthrin). **Explanation:** **1. Why Pyrethroids are the Correct Choice:** Pyrethroids (e.g., **Deltamethrin, Cyfluthrin, Alpha-cypermethrin**) are the only class of insecticides currently recommended by the WHO for treating Long-Lasting Insecticidal Nets (LLINs). They are preferred because of their unique combination of **high efficacy** against mosquitoes, **rapid knockdown effect** (excito-repellent property), and **low mammalian toxicity**. They are stable enough to remain effective on the fabric even after multiple washes (usually up to 20 washes or 3 years). **2. Analysis of Incorrect Options:** * **B. Diethyltoluamide (DEET):** This is a chemical **repellent** applied to the skin or clothes to prevent bites; it does not kill insects and is not used for treating bed nets. * **C. Natural Pyrethrins:** While derived from the Chrysanthemum flower and insecticidal, they are highly **unstable** when exposed to light and heat (photo-labile), making them unsuitable for long-term use on bed nets. * **D. Organophosphates (e.g., Malathion):** These are primarily used for **Indoor Residual Spraying (IRS)** or space spraying (fogging). They are generally too toxic for the close, prolonged skin contact associated with sleeping under a bed net. **3. NEET-PG High-Yield Pearls:** * **LLIN vs. ITBN:** LLINs (Long-Lasting Insecticidal Nets) are the current standard; they are factory-treated and do not require re-treatment, unlike older ITBNs. * **The "Halo Effect":** Large-scale use of ITBNs provides community-wide protection by reducing the overall mosquito population, not just protecting the individual sleeper. * **Target:** ITBNs are a core strategy for **Malaria** and **Lymphatic Filariasis** control. * **Mechanism:** Pyrethroids work by prolonging the opening of sodium channels in the insect's nervous system, leading to paralysis and death.

Question 1

Which of the following is NOT true about diphtheria?

Accepted Answer

Chemoprophylaxis is done with rifampicin.

Answer

Incubation period is 2 to 5 days.

Answer

Previously immunized asymptomatic household contacts should receive a booster dose.

Answer

A child with diphtheria is given antitoxin.

Question 2

Which of the following diseases is NOT transmitted by Aedes aegypti?

Accepted Answer

Filariasis

Answer

Yellow fever

Answer

Dengue fever

Answer

Chikungunya Hemorrhagic fever

Question 3

In the National Malaria Eradication Programme (NMEP), which of the following is NOT a recommended strategy for an area with high Annual Parasite Index (API)?

Accepted Answer

Follow-up of every case for one year and monthly blood smears

Answer

Presumptive treatment of all fever cases

Answer

Regular DDT spraying throughout the year

Answer

Epidemiological investigation of all malaria cases

Question 4

What is the vector for Yellow fever?

Accepted Answer

Aedes

Answer

Culex

Answer

Anopheles

Answer

Mansonoides

Question 5

Which of the following diseases is spread by mosquitoes?

Accepted Answer

Dengue

Answer

Kala azar

Answer

Trypanosomiasis

Answer

Listeriosis

Question 6

Which vector is most dangerous for transmitting plague?

Accepted Answer

Partially blocked flea

Answer

Blocked flea

Answer

Both

Answer

Unblocked flea

Question 7

Which of the following is NOT a zoonotic disease?

Accepted Answer

Giardiasis

Answer

Leptospirosis

Answer

Brucellosis

Answer

Rabies

Question 8

The Aedes aegypti index should be less than what percentage?

Accepted Answer

1%

Answer

5%

Answer

8%

Answer

10%

Question 9

A malarial survey is conducted in 50 villages having a population of one lakh. Out of 20,000 slides examined, 500 turned out to be malaria positive. What is the annual parasite incidence?

Accepted Answer

0.50%

Answer

20%

Answer

5%

Answer

0.40%

Question 10

Insecticide-treated bed nets (ITBNs) are typically treated with which class of insecticides?

Accepted Answer

Pyrethroids (e.g., Deltamethrin, Cyfluthrin)

Answer

Diethyltoluamide (DEET)

Answer

Natural Pyrethrins

Answer

Organophosphates (e.g., Malathion)

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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