Infectious Diseases — MCQs

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 101: All of the following are transmitted by Aedes mosquitoes, except?

A. Yellow fever
B. Dengue
C. Chikungunya
D. None of the above (Correct Answer)

Explanation: ### Explanation The correct answer is **D. None of the above**, because all three diseases listed (Yellow fever, Dengue, and Chikungunya) are primarily transmitted by the **Aedes** genus of mosquitoes, specifically *Aedes aegypti* and *Aedes albopictus*. #### Analysis of Options: * **Yellow Fever:** This is a viral hemorrhagic fever caused by a Flavivirus. In urban cycles, it is transmitted to humans by the bite of infected *Aedes aegypti*. * **Dengue:** Caused by the Dengue virus (DENV 1-4), it is the most common mosquito-borne viral disease in humans. *Aedes aegypti* is the primary vector, known for being a "day-biter" and breeding in clean, stagnant water. * **Chikungunya:** This is caused by an Alphavirus. Like Dengue, it is transmitted by *Aedes aegypti* and *Aedes albopictus*. It is clinically characterized by high fever and severe, often lingering, joint pain. Since all three diseases are transmitted by Aedes, none of the individual options can be the "exception." #### High-Yield Clinical Pearls for NEET-PG: * **Vector Characteristics:** *Aedes aegypti* is a "nervous feeder" (bites multiple people to complete one meal) and is a **day-biter** (peak activity at sunrise and sunset). * **Breeding Habit:** They are "container breeders," preferring artificial collections of clean water (e.g., desert coolers, flower pots, discarded tires). * **Other Aedes-transmitted diseases:** Zika virus and Rift Valley Fever. * **The "Tiger Mosquito":** *Aedes albopictus* is known as the Asian Tiger mosquito due to its striped appearance and is a significant secondary vector for these diseases.

Question 102: Which of the following diseases is under WHO surveillance?

A. Rabies (Correct Answer)
B. Mumps
C. Hepatitis
D. Tetanus

Explanation: **Explanation:** The World Health Organization (WHO) maintains a global surveillance system to monitor diseases that pose a significant public health threat, have potential for international spread, or require coordinated global control efforts. **Why Rabies is Correct:** Rabies is a fatal zoonotic disease included under the **WHO’s list of diseases under global surveillance**. It is a priority because it is 100% vaccine-preventable yet has a near 100% case-fatality rate. The WHO, along with the FAO and OIE, has launched the "Zero by 30" global strategic plan to end human deaths from dog-mediated rabies by 2030, necessitating rigorous surveillance and reporting. **Analysis of Incorrect Options:** * **Mumps:** While a common childhood vaccine-preventable disease, it is monitored by national immunization programs but is not part of the core WHO global surveillance mandate for international health security. * **Hepatitis:** While WHO has a global strategy for Viral Hepatitis (B and C), it is generally managed through regional control programs rather than the specific "International Health Regulations (IHR)" surveillance list. * **Tetanus:** Although Maternal and Neonatal Tetanus (MNT) elimination is a global goal, tetanus is not a communicable disease (it does not spread person-to-person) and therefore does not require the same type of international outbreak surveillance as Rabies. **High-Yield NEET-PG Pearls:** * **WHO Surveillance Categories:** Historically divided into **Quarantinable diseases** (Cholera, Plague, Yellow Fever) and **Diseases under International Surveillance** (Rabies, Influenza, Malaria, Polio, SARS, etc.). * **IHR (2005):** Requires countries to notify WHO of all cases of Wild Polio, Smallpox, SARS, and Human Influenza caused by a new subtype. * **Rabies Incubation:** Typically 1–3 months; characterized by Negri bodies (intracytoplasmic inclusions) in the hippocampus.

Question 103: Plague is what type of zoonosis?

A. Cyclozoonosis
B. Direct zoonosis
C. Sapro-zoonosis
D. Metazoonosis (Correct Answer)

Explanation: ### Explanation The classification of zoonoses is based on the life cycle of the infecting organism and the necessity of different hosts for its maintenance. **Why Metazoonosis is Correct:** **Metazoonoses** are diseases transmitted biologically by **invertebrate vectors**. In these diseases, the agent multiplies, develops, or both, within the vector before being transmitted to a vertebrate host. **Plague** (*Yersinia pestis*) is a classic metazoonosis because it requires the **rat flea (*Xenopsylla cheopis*)** as a biological vector to transmit the bacteria from rodents to humans. **Analysis of Incorrect Options:** * **Direct Zoonosis:** These are maintained in a single vertebrate species (e.g., Rabies, Brucellosis, Anthrax). They are transmitted from an infected vertebrate to a susceptible one by contact or a mechanical vector. * **Cyclozoonosis:** These require **more than one vertebrate host** species (but no invertebrate host) to complete the agent’s life cycle. Examples include *Taenia solium* (requires humans and pigs) and Echinococcosis. * **Sapro-zoonosis:** These require a **non-animal environmental reservoir** (like soil or decaying organic matter) for the development of the agent. Examples include Tetanus and Histoplasmosis. **High-Yield Clinical Pearls for NEET-PG:** * **Vector of Plague:** *Xenopsylla cheopis* (Rat flea) is the most efficient vector. * **Mechanism:** The "Blocked Flea" phenomenon occurs when *Y. pestis* multiplies in the flea's proventriculus, preventing blood intake and forcing the flea to regurgitate bacteria into the host. * **Indicators:** The **Flea Index** (specifically the Cheopis index >1) is a critical epidemiological marker for an impending plague outbreak. * **Drug of Choice:** Streptomycin is traditionally the DOC, though Gentamicin and Doxycycline are also used.

Question 104: Which of the following is a rabies-free country?

A. China
B. Australia (Correct Answer)
C. France
D. Russia

Explanation: **Explanation:** **Correct Answer: B. Australia** **Medical Concept:** A "rabies-free" country is defined by the World Health Organization (WHO) as a territory where no indigenous human or animal cases of rabies (caused by the classical *Rabies virus*) have been reported for at least two years, supported by a rigorous surveillance system and strict import/quarantine laws. Australia is historically recognized as rabies-free. While the **Australian Bat Lyssavirus (ABLV)** exists in local bat populations and can cause a rabies-like illness, the classical rabies virus (RABV) is not endemic to the continent. **Analysis of Incorrect Options:** * **A. China:** China remains one of the highest-burden countries for human rabies globally, primarily transmitted through domestic dogs. * **C. France:** While France has eliminated rabies in its terrestrial wildlife (like foxes) through oral vaccination programs, it is not classified as entirely rabies-free due to occasional imported cases and the presence of European Bat Lyssaviruses. * **D. Russia:** Rabies is endemic across Russia, with significant reservoirs in wild foxes, wolves, and stray dogs. **High-Yield NEET-PG Pearls:** * **Rabies-Free Areas:** Other notable rabies-free regions include the **United Kingdom, Japan, New Zealand, Iceland, and parts of Oceania.** * **Island Status:** Most rabies-free nations are islands, which allows for strict control over animal movement. * **India Context:** India has the highest incidence of rabies in the world (approx. 20,000 deaths/year). **Lakshadweep and the Andaman & Nicobar Islands** are the only parts of India considered rabies-free. * **Rule of 100:** Rabies is considered **100% fatal** once clinical symptoms appear, but **100% preventable** with timely Post-Exposure Prophylaxis (PEP).

Question 105: The human is the dead-end host for which of the following organisms?

A. Malaria
B. Typhoid
C. Bubonic plague (Correct Answer)
D. Filaria

Explanation: ### Explanation **Correct Answer: C. Bubonic plague** In medical ecology, a **dead-end host** (or accidental host) is an organism from which an infectious agent cannot be transmitted to another susceptible host. This usually occurs because the pathogen does not reach a high enough concentration in the blood (bacteremia/viremia) or is not shed in sufficient quantities for a vector or environment to pick it up. In **Bubonic plague** (*Yersinia pestis*), the natural cycle involves rodents and fleas. Humans are infected via the bite of an infected rat flea (*Xenopsylla cheopis*). In the bubonic form, the bacteria proliferate in the lymph nodes (buboes) but do not typically reach high enough levels in the blood to infect a new flea. Therefore, the human cannot sustain the transmission cycle. *(Note: If the infection progresses to Pneumonic plague, humans can transmit it via respiratory droplets, but for the Bubonic form, they remain a dead-end host.)* **Why the other options are incorrect:** * **A. Malaria:** Humans are an essential part of the life cycle. The *Anopheles* mosquito picks up gametocytes from human blood to continue the cycle. * **B. Typhoid:** Humans are the **only** reservoir for *Salmonella typhi*. Transmission occurs via the feco-oral route from cases or chronic carriers. * **D. Filaria:** Humans are the definitive host. The *Culex* mosquito ingests microfilariae from human blood to facilitate the parasite's development. **High-Yield Clinical Pearls for NEET-PG:** * **Other Dead-end Hosts:** Humans are also dead-end hosts for **Japanese Encephalitis (JE)**, **Rabies**, **Hydatid disease** (*Echinococcus granulosus*), and **Toxoplasmosis**. * **Plague Vector:** The most efficient vector is *Xenopsylla cheopis* (Rat flea). * **Indicator:** A "Rat Fall" is a classic warning sign of an impending plague outbreak in a community.

Question 106: Which of the following statements regarding leprosy is false?

A. Multibacillary leprosy is defined as a patient having more than 5 skin lesions.
B. A defaulter is defined as a patient who has not collected treatment for 6 months. (Correct Answer)
C. The target for elimination of leprosy is a prevalence rate below 1 per 10,000 population.
D. Annual New Case Detection Rate (ANCDR) is a measure of the incidence of leprosy.

Explanation: ### Explanation **1. Why Option B is the Correct Answer (False Statement):** Under the National Leprosy Eradication Programme (NLEP) and WHO guidelines, the definition of a **defaulter** (now often referred to as "Lost to Follow-up") has been updated. A patient is considered a defaulter if they have not collected treatment for **more than 12 consecutive months** for Multibacillary (MB) leprosy or **more than 6 consecutive months** for Paucibacillary (PB) leprosy. The blanket statement of "6 months" is therefore incorrect in the current clinical context. **2. Analysis of Other Options:** * **Option A:** According to the WHO operational classification, leprosy is classified as **Multibacillary (MB)** if there are **>5 skin lesions**, or if there is involvement of >1 nerve trunk, or a positive skin smear. * **Option C:** The **Elimination of Leprosy** as a public health problem is defined as achieving a prevalence rate of **less than 1 case per 10,000 population**. (Note: Eradication refers to zero cases globally). * **Option D:** Since leprosy has a long incubation period, the **Annual New Case Detection Rate (ANCDR)** is used as a proxy for **incidence**. It reflects the transmission of the disease in the community and the effectiveness of the surveillance system. **3. High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Average 3–5 years (shortest for PB, longest for MB). * **Most Sensitive Indicator:** ANCDR is the best indicator to monitor the impact of leprosy control programs. * **Prevalence Rate:** Calculated as (Number of cases on RFT at the end of the year / Total Population) × 10,000. * **MDT Regimen (Adult MB):** Rifampicin (600mg once monthly), Dapsone (100mg daily), and Clofazimine (300mg once monthly + 50mg daily) for **12 months**.

Question 107: According to WHO, how is tuberculosis detected?

A. Elisa
B. Mass Miniature Radiography
C. Sputum examination (Correct Answer)
D. Mantoux test

Explanation: **Explanation:** The gold standard and primary method for detecting pulmonary tuberculosis (PTB) according to WHO and the National TB Elimination Program (NTEP) is **Sputum Examination**. This includes both **Sputum Smear Microscopy** (using Ziehl-Neelsen staining) and **Molecular Testing** (such as CBNAAT/GeneXpert). Sputum examination is preferred because it provides definitive evidence of the presence of *Mycobacterium tuberculosis* and, in the case of microscopy, identifies "infectious" cases (open cases) that pose a public health risk. **Analysis of Incorrect Options:** * **A. ELISA:** Serological tests (ELISA) for TB are **banned** by the WHO and the Government of India. They are inconsistent, have low sensitivity/specificity, and lead to many false positives/negatives. * **B. Mass Miniature Radiography (MMR):** While Chest X-rays are excellent screening tools, they are not diagnostic on their own as they cannot differentiate between active TB, healed TB, or other lung pathologies. MMR is no longer recommended for mass screening due to high costs and radiation risks. * **C. Mantoux Test (TST):** This is a test of **infection, not disease**. A positive result only indicates that the individual has been exposed to the tubercle bacilli at some point; it does not confirm active clinical tuberculosis. **High-Yield Clinical Pearls for NEET-PG:** * **CBNAAT (GeneXpert):** Now the preferred first-line diagnostic test under NTEP for all presumptive TB cases. It detects both *M. tuberculosis* and Rifampicin resistance. * **Sputum Culture (LJ Medium):** The absolute "Gold Standard" for diagnosis, though it takes 6–8 weeks for results. * **Infectivity:** A single positive sputum smear is sufficient to initiate treatment under current guidelines.

Question 108: Which of the following is true about resistant tuberculosis?

A. Sputum smear is positive after 5 months.
B. Culture is positive after 3 months.
C. Clinical deterioration. (Correct Answer)
D. Active disease in chest X-ray.

Explanation: In the context of Tuberculosis management under the National TB Elimination Program (NTEP), identifying treatment failure or resistance is crucial. **Explanation of the Correct Answer:** **Clinical deterioration** (Option C) is a primary indicator of resistant tuberculosis. When a patient is on an appropriate weight-band-based regimen but continues to show worsening symptoms (e.g., persistent fever, significant weight loss, or worsening cough), it strongly suggests that the *Mycobacterium tuberculosis* strain is not responding to the current drugs, likely due to drug resistance (MDR/XDR-TB). **Analysis of Incorrect Options:** * **Option A (Sputum smear positive after 5 months):** This is the classical definition of **Treatment Failure** in older guidelines. While it indicates resistance, the question asks for what is "true" regarding the identification of resistance. Modern protocols prioritize molecular testing (CBNAAT) much earlier. * **Option B (Culture positive after 3 months):** In the current NTEP algorithm, if a patient remains smear-positive at the end of the Intensive Phase (2 months), they are immediately screened for drug resistance using molecular methods. Waiting for a 3-month culture is not the standard diagnostic criterion for resistance. * **Option D (Active disease in chest X-ray):** Radiological findings often lag behind clinical and microbiological improvement. A "static" or "active" X-ray alone does not confirm resistance, as lesions take time to heal. **High-Yield Clinical Pearls for NEET-PG:** * **MDR-TB:** Resistance to at least **Isoniazid (H)** and **Rifampicin (R)**. * **XDR-TB:** MDR-TB plus resistance to any **Fluoroquinolone** and at least one **Group A drug** (Bedaquiline or Linezolid) as per the 2020 WHO updated definitions. * **Diagnostic Gold Standard:** **CBNAAT (GeneXpert)** is the initial diagnostic tool for all presumptive TB cases to rule out Rifampicin resistance at the outset. * **Universal Drug Susceptibility Testing (UDST):** Every TB patient in India should be tested for drug resistance at the time of diagnosis.

Question 109: Which of the following is NOT a zoonotic disease?

A. Tuberculosis
B. Q fever
C. Taeniasis
D. Rubella (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the definition of a **Zoonosis**, which is an infectious disease that is naturally transmitted from vertebrate animals to humans. **Why Rubella is the correct answer:** Rubella (German Measles) is **not** a zoonotic disease because humans are the **only known reservoir** for the Rubella virus. It is transmitted person-to-person via respiratory droplets. Since there is no animal involvement in its natural life cycle, it cannot be classified as a zoonosis. **Analysis of incorrect options:** * **Tuberculosis (A):** While *M. tuberculosis* is primarily human-to-human, *Mycobacterium bovis* causes **Bovine Tuberculosis**, which is a classic zoonosis transmitted to humans via unpasteurized milk. * **Q Fever (B):** Caused by *Coxiella burnetii*, this is a highly infectious zoonosis. The primary reservoirs are cattle, sheep, and goats. Humans are infected by inhaling contaminated dust or handling infected animal products. * **Taeniasis (C):** This is a parasitic zoonosis. *Taenia saginata* (beef tapeworm) and *Taenia solium* (pork tapeworm) require animal intermediate hosts to complete their life cycle before infecting humans. **NEET-PG High-Yield Pearls:** 1. **Anthropozoonoses:** Diseases transmitted from animals to man (e.g., Rabies, Leptospirosis, Plague). 2. **Zooanthroponoses:** Diseases transmitted from man to animals (e.g., Human Tuberculosis to cattle). 3. **Cyclozoonoses:** Require more than one vertebrate host species to complete the life cycle (e.g., Echinococcosis, Taeniasis). 4. **Metazoonoses:** Transmitted by an invertebrate vector (e.g., Plague, Arboviruses). 5. **Saprozoonoses:** Require a non-animal environmental reservoir like soil (e.g., Tetanus, Histoplasmosis).

Question 110: What is the percentage of HIV infection in a child born to an HIV-positive mother?

A. 20-30% (Correct Answer)
B. 10-20%
C. 70-80%
D. 100%

Explanation: **Explanation:** The risk of **Mother-to-Child Transmission (MTCT)** of HIV, in the absence of any medical intervention (antiretroviral therapy, elective cesarean section, or avoidance of breastfeeding), typically ranges between **20% and 30%**. Transmission can occur at three stages: 1. **In-utero (Transplacental):** ~5–10% 2. **During Labor/Delivery (Parturition):** ~10–15% (The most common period for transmission) 3. **Post-partum (Breastfeeding):** ~5–20% **Analysis of Options:** * **Option A (20-30%):** This is the standard global estimate for vertical transmission without intervention. In non-breastfeeding populations, the risk is roughly 15-25%; in breastfeeding populations, it rises to 30-45%. * **Option B (10-20%):** This range is too low for natural transmission rates but may be seen if partial interventions (like single-dose Nevirapine) are used. * **Option C & D (70-80% and 100%):** These are incorrect as HIV is not transmitted to every fetus. The placental barrier and maternal immune factors provide significant protection. **High-Yield NEET-PG Pearls:** * **PPTCT Goal:** With Effective Antiretroviral Therapy (ART) and viral suppression, the risk of transmission can be reduced to **less than 2%**. * **Most common route:** Intrapartum (during delivery) due to contact with infected maternal blood and vaginal secretions. * **Diagnosis in Infants:** HIV DNA PCR is the gold standard (at 6 weeks). Antibody tests (ELISA) are unreliable until 18 months due to the persistence of maternal IgG antibodies. * **Prophylaxis:** Under National guidelines, the infant receives Nevirapine syrup for 6 weeks (extendable to 12 weeks if the mother is not virally suppressed).

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