Infectious Diseases Practice Questions

Q: What is the incubation period of typhoid?

10-14 days. **Explanation:** **Typhoid fever**, caused by the bacterium *Salmonella Typhi*, is a systemic infection characterized by a prolonged incubation period. The correct answer is **10-14 days**, though it can range from 3 to 21 days depending on the infectious dose. **Why 10-14 days is correct:** The pathogenesis of typhoid involves a complex "step-ladder" progression. After ingestion, the bacilli penetrate the intestinal mucosa and are sequestered within the mesenteric lymph nodes. They undergo primary multiplication within the reticuloendothelial system (liver, spleen, and bone marrow). The incubation period represents the time required for the bacteria to multiply sufficiently to cause a **secondary bacteremia**, at which point clinical symptoms like fever and malaise manifest. **Analysis of Incorrect Options:** * **A. 1-2 days:** This is too short for a systemic infection like typhoid. Such short incubation periods are typical of localized bacterial gastroenteritis (e.g., *Salmonella Enteritidis* or *Vibrio cholerae*). * **C. 1 month:** While rare cases can extend to 3 weeks, a 1-month incubation is atypical for *S. Typhi* and more characteristic of infections like Hepatitis A or certain parasitic infestations. **Clinical Pearls for NEET-PG:** * **Transmission:** Fecal-oral route; humans are the only natural reservoir. * **Diagnosis (WAT):** **W**idal test (positive in 2nd week), **A**ntigen detection, and **T**yphidot. * **Gold Standard:** Bone marrow culture is the most sensitive, but **Blood culture** is the investigation of choice in the **1st week**. * **Carrier State:** The **gallbladder** is the most common site of chronic colonization (e.g., the famous case of Typhoid Mary).

Q: Which of the following statements are true about Cholera?

Food can transmit cholera. **Explanation:** **Cholera** is an acute diarrheal infection caused by the ingestion of food or water contaminated with the bacterium *Vibrio cholerae*. 1. **Why Option A is Correct:** Cholera is primarily transmitted via the **fecal-oral route**. While contaminated water is the most common vehicle, **food** (especially raw or undercooked shellfish, contaminated fruits, and vegetables) acts as a significant reservoir for transmission. In endemic areas, "street foods" and moist grains are frequent sources of outbreaks. 2. **Why Other Options are Incorrect:** * **Option B:** Modern Oral Cholera Vaccines (OCVs) like Shanchol and Dukoral provide significant protection, but their efficacy is generally around **60–80%**, not 90%. Protection also wanes significantly after 2–3 years. * **Option C:** In Cholera, **"Healthy carriers" do not exist** in the epidemiological sense of long-term transmission. While "Asymptomatic cases" occur (and outnumber clinical cases), the term "Healthy carrier" usually refers to chronic carriage (like in Typhoid). In Cholera, the carrier state is transient (incubatory or convalescent), usually lasting less than 2–4 weeks. * **Option D:** *Vibrio cholerae* is highly sensitive to chlorine. **Chlorination** of water supplies (maintaining a free residual chlorine level of 0.5 mg/L) is one of the most effective public health interventions to control epidemics. **High-Yield NEET-PG Pearls:** * **Rice Water Stools:** The hallmark clinical sign. * **Haldane’s Rule:** The ratio of asymptomatic to symptomatic cases is high (approx. 10:1 to 100:1 for El Tor biotype). * **Darting Motility:** Observed under dark-ground microscopy. * **Treatment of Choice:** Prompt rehydration (ORS/IV fluids) and **Doxycycline** (drug of choice to reduce stool volume and duration).

Q: During a cholera epidemic, what is the first step to be taken?

Ensure a safe water supply and sanitation. **Explanation:** In the management of a cholera epidemic, the immediate priority is to break the chain of transmission. Since *Vibrio cholerae* is primarily transmitted via the fecal-oral route through contaminated water and food, **ensuring a safe water supply and sanitation** is the most effective first step to prevent further spread. This involves chlorination of water sources, health education regarding hand hygiene, and proper disposal of excreta. **Analysis of Options:** * **Option B (Vaccination):** While oral cholera vaccines (OCV) are used for prevention in endemic areas, they are not the "first step" during an active epidemic. They take time to provide immunity and do not address the immediate source of infection. * **Option C & D (Chemoprophylaxis/Tetracycline):** Mass chemoprophylaxis (treating everyone) is **not recommended** by the WHO. It is ineffective in stopping an epidemic, leads to antibiotic resistance, and provides a false sense of security. Selective chemoprophylaxis is only considered for close household contacts. **NEET-PG High-Yield Pearls:** * **Gold Standard Treatment:** Rehydration (ORS and IV fluids) is the mainstay of clinical management to reduce mortality. * **Drug of Choice:** **Doxycycline** (single dose) is the drug of choice for treating cases and for selective chemoprophylaxis. * **Indicator of Water Safety:** For drinking water during an epidemic, the **residual chlorine** level should be **0.5 mg/L**. * **Best Method of Water Disinfection:** Chlorination is the most practical and effective method during outbreaks.

Q: Which arthropod is described by the following features: bilateral compressed and wingless body, conical head, short piercing mouthparts, and three pairs of strong legs?

Rat flea. ### Explanation The description provided is characteristic of the **Rat flea (*Xenopsylla cheopis*)**. **1. Why the Correct Answer is Right:** * **Bilaterally compressed body:** This unique anatomical feature allows fleas to move swiftly between the hairs or feathers of their hosts. * **Wingless and Strong Legs:** Fleas are secondarily wingless. Their third pair of legs is particularly powerful and specialized for jumping long distances relative to their size. * **Conical Head and Piercing Mouthparts:** These are adaptations for their ectoparasitic lifestyle, allowing them to pierce the host's skin and feed on blood. **2. Why the Incorrect Options are Wrong:** * **Hard and Soft Ticks (Options B & C):** Ticks belong to the class Arachnida. They have a dorso-ventrally flattened body (not bilaterally compressed) and adults possess four pairs of legs, not three. Hard ticks have a scutum (dorsal shield), which is absent in soft ticks. * **Mites (Option D):** Mites are microscopic arachnids. Like ticks, they have four pairs of legs as adults and lack the distinct "compressed" body shape and jumping legs of a flea. **3. High-Yield Clinical Pearls for NEET-PG:** * **Vector Potential:** The Rat flea is the primary vector for **Bubonic Plague** (*Yersinia pestis*) and **Endemic (Murine) Typhus** (*Rickettsia typhi*). * **The "Blocked Flea" Phenomenon:** *Y. pestis* multiplies in the flea's proventriculus, creating a charcoal-like plug. This "blocked flea" becomes hungry and regurgitates bacteria into the next host during feeding attempts. * **Flea Index:** A "Rat Flea Index" > 1 is considered a critical threshold for a potential plague outbreak in a community. * **Control:** Use insecticides (like DDT or Malathion) to kill fleas *before* using rodenticides, to prevent hungry fleas from leaving dead rats and biting humans.

Q: Japanese Encephalitis is transmitted by which mosquito species?

All of the above. **Explanation:** Japanese Encephalitis (JE) is the leading cause of viral encephalitis in Asia, caused by a Flavivirus. It is a zoonotic disease where the virus is maintained in an **enzootic cycle** involving mosquitoes as vectors and pigs or water birds (Ardeid birds) as reservoirs. **Why "All of the above" is correct:** The primary vectors for Japanese Encephalitis are mosquitoes of the **Culex vishnui subgroup**. This subgroup consists of three main species that are all competent vectors: 1. **Culex tritaeniorhynchus:** This is the **most important and principal vector** across most of Asia, including India. It breeds predominantly in stagnant water like rice fields. 2. **Culex vishnui:** A significant vector that contributes to transmission in rural areas. 3. **Culex pseudovishnui:** Also a proven vector within the same subgroup. Since all three species belong to the primary vector group responsible for the transmission of the JE virus to humans, option D is the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **Reservoirs:** Pigs are considered "amplifier hosts" (they develop high viremia), while Ardeid birds (herons, egrets) are the natural reservoir hosts. * **Dead-end Hosts:** Humans and horses are "dead-end hosts" because they do not develop sufficient viremia to infect feeding mosquitoes. * **Seasonality:** Transmission usually peaks during the rainy season and pre-harvest periods (due to rice field irrigation). * **Diagnosis:** The investigation of choice is **MAC-ELISA** to detect IgM antibodies in CSF or serum. * **Vaccination:** The **SA-14-14-2** (live attenuated) vaccine is commonly used in the Universal Immunization Programme (UIP) in endemic areas of India.

Q: What is the duration of infectivity for a patient with diphtheria?

For 15 days after infection. **Explanation:** The duration of infectivity for **Diphtheria** (caused by *Corynebacterium diphtheriae*) typically lasts until the bacilli have disappeared from the secretions, which usually takes **14 to 28 days** (averaging 15 days) from the onset of the disease. 1. **Why Option D is Correct:** In an untreated case, a patient is considered infectious for approximately **2 to 4 weeks**. For examination purposes, **15 days** is the standard benchmark used to define the period of communicability. However, with appropriate antibiotic therapy (Penicillin or Erythromycin), the patient usually becomes non-infectious within 24–48 hours. 2. **Why Other Options are Incorrect:** * **Option A:** Diphtheria is characterized by a "barking" cough (croup) due to the pseudomembrane, but infectivity is linked to the presence of the bacilli in the throat/nasopharynx, not the clinical symptom of coughing. * **Option B:** Fever in diphtheria is usually low-grade. Infectivity persists long after the patient becomes afebrile. * **Option C:** Diphtheria is an acute infection. While "chronic carriers" exist, the disease itself is not lifelong. **High-Yield NEET-PG Pearls:** * **Schick Test:** Used to demonstrate immunity/susceptibility to Diphtheria. * **Virulence Test:** Elek's gel precipitation test is used to detect toxin production. * **Culture Media:** Loffler’s Serum Slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Case Definition:** Presence of a greyish-white **pseudomembrane** (cannot be easily scraped off; bleeds on touch). * **Contact Management:** All contacts should be kept under surveillance for 7 days and given a prophylactic dose of Erythromycin.

Q: What is an important indicator to measure the incidence of malaria in a community?

Annual parasite incidence. **Explanation:** **Annual Parasite Incidence (API)** is the most sensitive and widely used indicator to measure the **incidence** of malaria in a community. It represents the number of confirmed malaria cases per 1,000 population per year. Since "incidence" refers to the occurrence of new cases, API—which tracks laboratory-confirmed cases over a specific timeframe—is the gold standard for monitoring the malaria burden and the effectiveness of control programs. **Analysis of Options:** * **Slide Positivity Rate (SPR):** This measures the percentage of blood slides found positive among the total slides examined. While useful for monitoring trends, it is heavily dependent on the total number of slides collected (fever surveillance) rather than the actual population at risk. * **Annual Falciparum Incidence (AFI):** This specifically measures the incidence of *P. falciparum* cases. While critical for assessing the severity of the malaria situation (as falciparum is more lethal), it does not represent the total incidence of all malaria species (e.g., *P. vivax*). * **Infant Parasite Rate (IPR):** This measures the prevalence of malaria parasites in infants (below 1 year). It is considered the best indicator for measuring **recent transmission** in a locality, as infants are unlikely to have traveled or had long-standing infections. **High-Yield Facts for NEET-PG:** * **API Formula:** (Total confirmed cases in a year / Total population under surveillance) × 1000. * **Target:** Under the National Framework for Malaria Elimination, an API of **<1 per 1,000 population** is the threshold for moving into the elimination phase. * **Annual Blood Examination Rate (ABER):** Measures the efficiency of surveillance; it should ideally be **10% or more**. * **Indicator of Endemicity:** Spleen rate in children (2–9 years) is traditionally used to classify the endemicity of malaria in an area.

Q: Which of the following is NOT a vector-borne disease?

Tetanus. **Explanation:** The correct answer is **Tetanus** because it is a **soil-borne infection**, not a vector-borne disease. **1. Why Tetanus is the correct answer:** Tetanus is caused by the bacterium *Clostridium tetani*. It is transmitted through the contamination of wounds (such as punctures, lacerations, or animal bites) with bacterial spores found in **soil, dust, and animal feces**. It does not require an arthropod vector (like a mosquito or louse) for transmission. **2. Why the other options are incorrect:** * **Epidemic Typhus:** This is a vector-borne disease caused by *Rickettsia prowazekii* and is transmitted by the **Human Body Louse** (*Pediculus humanus corporis*). * **Japanese Encephalitis (JE):** This is a viral zoonotic disease transmitted to humans by the bite of infected **Culex mosquitoes** (primarily *Culex tritaeniorhynchus*). * **Dengue:** This is a viral infection transmitted to humans through the bite of infected **Aedes mosquitoes** (primarily *Aedes aegypti*). **High-Yield Clinical Pearls for NEET-PG:** * **Tetanus:** Often presents with "Lockjaw" (Trismus) and "Risus Sardonicus" (characteristic grin due to facial muscle spasms). The toxin involved is **Tetanospasmin**, which blocks inhibitory neurotransmitters (GABA and Glycine). * **Vector Identification:** * *Aedes:* Day biter, breeds in artificial collections of clean water. * *Culex:* Night biter, breeds in dirty/stagnant water (paddy fields for JE). * **Epidemic vs. Endemic Typhus:** Epidemic typhus is louse-borne, while Endemic (Murine) typhus is flea-borne.

Question 1

What is the incubation period of typhoid?

Accepted Answer

10-14 days

Answer

1-2 days

Answer

1 month

Answer

None of the above

Question 2

Which of the following statements are true about Cholera?

Accepted Answer

Food can transmit cholera

Answer

Vaccination gives 90% efficiency

Answer

Healthy carriers can transmit cholera

Answer

Chlorination is not effective

Question 3

During a cholera epidemic, what is the first step to be taken?

Accepted Answer

Ensure a safe water supply and sanitation

Answer

Administer cholera vaccination to all individuals

Answer

Provide primary chemoprophylaxis

Answer

Treat everyone with tetracycline

Question 4

Which arthropod is described by the following features: bilateral compressed and wingless body, conical head, short piercing mouthparts, and three pairs of strong legs?

Accepted Answer

Rat flea

Answer

Soft tick

Answer

Hard tick

Answer

Mite

Question 5

A person is considered as a case of tuberculosis if which of the following conditions are met?

Accepted Answer

Sputum positive

Answer

Has cough

Answer

X-ray signs suggestive of tuberculosis

Answer

Mantoux test positive

Question 6

Japanese Encephalitis is transmitted by which mosquito species?

Accepted Answer

All of the above

Answer

Culex tritaeniorhynchus

Answer

Culex vishnui

Answer

Culex pseudovishnui

Question 7

What is the duration of infectivity for a patient with diphtheria?

Accepted Answer

For 15 days after infection

Answer

Until the cough subsides

Answer

Until the patient is febrile

Answer

Lifelong

Question 8

What is an important indicator to measure the incidence of malaria in a community?

Accepted Answer

Annual parasite incidence

Answer

Slide positivity rate

Answer

Annual falciparum incidence

Answer

Infant parasite rate

Question 9

Which of the following is NOT a vector-borne disease?

Accepted Answer

Tetanus

Answer

Epidemic typhus

Answer

Japanese encephalitis

Answer

Dengue

Question 10

For how long is a temporary carrier of typhoid infective?

Accepted Answer

3 months to 1 year

Answer

Less than 3 weeks

Answer

3 weeks to 3 months

Answer

More than 1 year

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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