Infectious Diseases — MCQs

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 41: Which of the following is a confirmatory test for Syphilis?

A. VDRL
B. Rapid plasma reagin test
C. Fluorescent treponemal antibody absorption (FT-ABS) test (Correct Answer)
D. All of the above

Explanation: **Explanation:** Syphilis, caused by *Treponema pallidum*, is diagnosed using two categories of serological tests: **Nonspecific (Nontreponemal)** and **Specific (Treponemal)**. **Why Option C is Correct:** The **Fluorescent Treponemal Antibody Absorption (FTA-ABS) test** is a specific treponemal test. It detects antibodies directed specifically against *T. pallidum* antigens. Because these tests have high specificity, they are used as **confirmatory tests** to verify a positive result from a screening test. Once positive, treponemal tests usually remain positive for life, regardless of treatment. **Why Other Options are Incorrect:** * **Options A (VDRL) and B (RPR):** These are **Nontreponemal tests**. They detect "reagin" antibodies (IgM and IgG) against cardiolipin-lecithin-cholesterol antigen. While they are excellent for **screening** and monitoring treatment response (as titers fall after successful therapy), they are prone to **Biological False Positives (BFP)** in conditions like SLE, leprosy, malaria, and pregnancy. Therefore, they are not confirmatory. * **Option D:** Incorrect because only treponemal tests serve as confirmatory tools. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Sequence:** The traditional algorithm starts with a nontreponemal test (VDRL/RPR) followed by a treponemal test (FTA-ABS or TPHA) for confirmation. * **TPHA (T. pallidum Hemagglutination Assay):** Another common confirmatory test, often preferred over FTA-ABS due to ease of use. * **Prozone Phenomenon:** Can cause a false-negative VDRL in secondary syphilis due to very high antibody titers; solved by diluting the serum. * **Neurosyphilis:** **CSF-VDRL** is the gold standard for diagnosis (highly specific, though low sensitivity).

Question 42: Which disease is transmitted by lice?

A. Epidemic typhus (Correct Answer)
B. Endemic typhus
C. Murine typhus
D. Rickettsial pox

Explanation: **Explanation:** The correct answer is **Epidemic typhus**. This disease is caused by *Rickettsia prowazekii* and is uniquely transmitted by the **human body louse** (*Pediculus humanus corporis*). The transmission occurs when louse feces containing the bacteria are rubbed into bite wounds or mucous membranes by scratching. **Analysis of Options:** * **Epidemic Typhus (A):** Transmitted by lice. It is characterized by high fever, headache, and a rash that spreads from the trunk to the extremities. It can recur years later as **Brill-Zinsser disease**. * **Endemic Typhus (B) & Murine Typhus (C):** These terms are synonymous. Both are caused by *Rickettsia typhi* and are transmitted to humans by the **rat flea** (*Xenopsylla cheopis*). * **Rickettsial pox (D):** Caused by *Rickettsia akari*, this disease is transmitted by **mites** associated with common house mice. It is clinically distinguished by an initial eschar at the bite site followed by a papulovesicular rash. **High-Yield NEET-PG Pearls:** * **Vector Mnemonic:** "Louse-borne is Lousy (Epidemic/Prowazekii)" vs. "Flea-borne is Endemic (Typhi)." * **Weil-Felix Test:** Epidemic and Endemic typhus both show a positive reaction with **OX-19**. * **Drug of Choice:** Doxycycline is the gold standard treatment for all rickettsial infections. * **Scrub Typhus:** Often confused with the above, it is caused by *Orientia tsutsugamushi* and transmitted by **trombiculid mites (chiggers)**; it shows a positive **OX-K** on Weil-Felix.

Question 43: Which of the following is NOT a sexually transmitted agent?

A. Streptococcus group B
B. Ebola
C. Candida Albicans
D. Coronavirus (Correct Answer)

Explanation: **Explanation:** The correct answer is **Coronavirus (SARS-CoV-2)**. While many pathogens can be found in various bodily fluids, a **Sexually Transmitted Infection (STI)** is defined by its primary or significant mode of transmission through sexual contact (vaginal, anal, or oral). **Why Coronavirus is the correct answer:** SARS-CoV-2 is primarily a **respiratory pathogen** transmitted via aerosols and respiratory droplets. While viral RNA has occasionally been detected in semen, there is currently no clinical evidence that sexual contact is a viable route of transmission. Therefore, it is not classified as a sexually transmitted agent. **Analysis of other options:** * **Streptococcus group B (S. agalactiae):** This is a common colonizer of the female genital tract. It is frequently transmitted sexually between partners and is a critical concern in obstetrics due to vertical transmission during birth. * **Ebola Virus:** This is a high-yield fact for NEET-PG. Ebola virus can persist in **immunologically privileged sites**, specifically the testes. It can be shed in **semen** for many months after clinical recovery, and documented cases of sexual transmission have occurred. * **Candida albicans:** While often considered an opportunistic overgrowth (commensal), it can be transmitted sexually, leading to conditions like balanoposthitis in male partners. **NEET-PG High-Yield Pearls:** * **Ebola Persistence:** The WHO recommends male survivors practice safe sex (condom use) for at least 12 months or until semen tests negative twice. * **Zika Virus:** Another emerging STI; it persists in semen longer than in blood. * **GBS Screening:** In pregnancy, screening for Group B Strep is typically done at 35–37 weeks of gestation to prevent neonatal sepsis/meningitis.

Question 44: Which of the following exposures carries the maximum risk of transmission of HIV?

A. Blood transfusion (Correct Answer)
B. Sexual intercourse
C. Needle prick
D. Transplacental

Explanation: The risk of HIV transmission depends on the route of exposure, the viral load in the source fluid, and the volume of inoculum. **Correct Option: A. Blood Transfusion** Blood transfusion carries the highest risk of HIV transmission, estimated at approximately **90–92% per exposure**. This is because a large volume of infected blood is introduced directly into the recipient's systemic circulation. When a unit of blood contaminated with HIV is transfused, seroconversion is almost certain. **Explanation of Incorrect Options:** * **B. Sexual Intercourse:** While this is the most common mode of transmission globally, the risk per single act is relatively low. Receptive anal intercourse carries the highest risk among sexual acts (~1.38%), followed by receptive vaginal intercourse (~0.08%). * **C. Needle Prick:** The average risk of HIV transmission after a percutaneous exposure to HIV-infected blood (e.g., accidental needlestick) is approximately **0.3%**. For mucous membrane exposure, it is even lower (~0.09%). * **D. Transplacental:** Mother-to-child transmission (MTCT) can occur during pregnancy, delivery, or breastfeeding. Without intervention, the risk ranges from **15% to 45%**; however, with effective Antiretroviral Therapy (ART), this can be reduced to <1%. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Risk (Highest to Lowest):** Blood Transfusion > Vertical Transmission (without ART) > Receptive Anal Sex > Needle Prick > Receptive Vaginal Sex. * **Post-Exposure Prophylaxis (PEP):** Should be started as soon as possible, ideally within **2 hours** and no later than **72 hours**, continuing for **28 days**. * **Standard PEP Regimen:** Tenofovir (300mg) + Lamivudine (300mg) + Dolutegravir (50mg) once daily.

Question 45: Which of the following is true about cerebral malaria in children?

A. Quinine is the drug of choice and hypoglycemia is a common complication. (Correct Answer)
B. Hypoglycemia is a common complication.
C. Cerebral malaria in children has a good prognosis.
D. Cerebral malaria is caused by Plasmodium vivax.

Explanation: ### Explanation **Correct Answer: A. Quinine is the drug of choice and hypoglycemia is a common complication.** Cerebral malaria is the most severe neurological complication of *Plasmodium falciparum* infection. In pediatric populations, **hypoglycemia** is a hallmark complication, occurring due to the parasite’s high glucose consumption, impaired hepatic gluconeogenesis, and the stimulatory effect of Quinine on pancreatic beta cells (hyperinsulinemia). While the WHO now recommends IV Artesunate as the first-line treatment for severe malaria globally, **Quinine** remains a classic "drug of choice" in many traditional exam patterns and is a vital alternative when artemisinins are unavailable. **Analysis of Incorrect Options:** * **Option B:** While hypoglycemia is indeed a common complication, Option A is a more comprehensive "best answer" as it addresses both management and pathophysiology. * **Option C:** Cerebral malaria in children carries a **guarded prognosis**. Even with treatment, the mortality rate remains high (15–20%), and survivors often suffer from long-term neurological sequelae such as hemiplegia, blindness, or epilepsy. * **Option D:** Cerebral malaria is almost exclusively caused by ***Plasmodium falciparum*** due to its ability to cause "sequestration" (cytoadherence of infected RBCs to capillary endothelium), leading to microvascular obstruction in the brain. *P. vivax* rarely causes such severe manifestations. **NEET-PG High-Yield Pearls:** * **Drug of Choice (Current WHO):** IV Artesunate (reduces mortality more effectively than Quinine). * **Quinine Side Effect:** Cinchonism (tinnitus, deafness, dizziness). * **Pathogenesis:** Mediated by **PfEMP-1** (P. falciparum erythrocyte membrane protein 1) which causes "rosetting" and sequestration. * **Poor Prognostic Markers:** Deep coma, peripheral schizontemia, and high lactate levels.

Question 46: What is the Kahn test?

A. A tube flocculation test. (Correct Answer)
B. A slide flocculation test.
C. A tube agglutination test.
D. A slide agglutination test.

Explanation: **Explanation:** The **Kahn test** is a non-specific (non-treponemal) serological test used for the diagnosis of Syphilis. It is classified as a **tube flocculation test**. 1. **Why Option A is correct:** In the Kahn test, the patient’s serum is mixed with a purified lipid antigen (cardiolipin) in a test tube. If antibodies (reagin) are present, they react with the antigen to form visible clumps or flakes that remain suspended in the liquid. This process is known as **flocculation**. Because this reaction is performed and observed in a tube, it is a tube flocculation test. 2. **Why other options are incorrect:** * **Option B:** A **slide flocculation test** describes the **VDRL** (Venereal Disease Research Laboratory) test, where the reaction is observed on a slide under a microscope. * **Options C & D:** **Agglutination** involves the clumping of particulate antigens (like bacteria or RBCs). Flocculation is a specific type of precipitation where the antigen is soluble but forms visible flakes. Tests like the Widal test are tube agglutination tests. **High-Yield Clinical Pearls for NEET-PG:** * **Antigen used:** The antigen in the Kahn test (and VDRL) is **Cardiolipin** (diphosphatidylglycerol), extracted from beef heart. * **Sensitivity:** The Kahn test is more sensitive than the Wassermann test (a complement fixation test) but has largely been replaced by the VDRL and RPR tests in modern practice. * **Biological False Positives (BFP):** Since these tests detect non-specific reagin antibodies, false positives can occur in conditions like SLE, leprosy, malaria, and pregnancy. * **Prozone Phenomenon:** High antibody titers can sometimes lead to a false-negative result in flocculation tests; this is corrected by diluting the serum.

Question 47: What caused the maximum damage to Napoleon's army during his march to Moscow?

A. Typhus (Correct Answer)
B. Plague
C. Diarrhea
D. Typhoid

Explanation: **Explanation:** The correct answer is **Typhus (Epidemic Typhus)**. Napoleon’s 1812 Russian campaign is a classic historical example of how infectious diseases can alter the course of war. Of the approximately 600,000 soldiers who entered Russia, only about 100,000 returned; the vast majority of these losses were due to Epidemic Typhus, not combat. **Why Typhus is Correct:** Epidemic Typhus is caused by ***Rickettsia prowazekii*** and is transmitted by the **human body louse (*Pediculus humanus corporis*)**. In the crowded, unsanitary, and cold conditions of the march, lice flourished. The bacteria are excreted in louse feces and enter the human body through skin abrasions caused by scratching. This led to high fevers, characteristic rashes, and massive mortality rates among the troops. **Why Incorrect Options are Wrong:** * **Plague:** Caused by *Yersinia pestis* (transmitted by rat fleas). While devastating in the Middle Ages (Black Death), it was not the primary driver of Napoleon’s losses. * **Diarrhea/Dysentery:** While common in military camps due to poor sanitation, it was a secondary cause of morbidity compared to the lethal epidemic of Typhus. * **Typhoid:** Caused by *Salmonella Typhi* (fecal-oral route). Though present, it did not reach the epidemic proportions or the high mortality rate seen with Typhus during this specific campaign. **High-Yield Clinical Pearls for NEET-PG:** * **Organism:** *Rickettsia prowazekii* (Obligate intracellular bacterium). * **Vector:** Human body louse (Note: The head louse is NOT a vector). * **Brill-Zinsser Disease:** A recrudescent (latent) form of Typhus occurring years after the primary infection. * **Weil-Felix Test:** A heterophile agglutination test used for diagnosis (though largely replaced by IFA). *R. prowazekii* reacts with **OX-19**. * **Drug of Choice:** Doxycycline.

Question 48: All of the following are sexually transmitted infections, except?

A. Candida albicans
B. Echinococcus (Correct Answer)
C. Molluscum contagiosum
D. Treponema pallidum

Explanation: **Explanation:** The correct answer is **Echinococcus (Option B)**. **1. Why Echinococcus is the correct answer:** *Echinococcus granulosus* is a cestode (tapeworm) responsible for **Hydatid disease**. It is transmitted via the **fecal-oral route**, specifically through the ingestion of food or water contaminated with eggs shed in the feces of definitive hosts (dogs). It is not transmitted through sexual contact. **2. Analysis of Incorrect Options:** * **Treponema pallidum (Option D):** This is the causative agent of **Syphilis**. It is a classic STI transmitted through direct contact with an infectious lesion (chancre) during sexual intercourse. * **Molluscum contagiosum (Option C):** Caused by a Poxvirus, this condition presents as umbilicated papules. While it can be spread via fomites in children, in adults, it is predominantly considered an **STI** when lesions are found in the anogenital region. * **Candida albicans (Option A):** While *Candida* is part of the normal vaginal flora and infections are often opportunistic (due to antibiotics or diabetes), it is medically classified as a **sexually transmissible infection** because it can be passed between partners during intercourse, leading to conditions like balanoposthitis in males. **Clinical Pearls for NEET-PG:** * **Hydatid Cyst:** Look for "Eggshell calcification" on X-ray and "Water lily sign" or "Hydatid sand" on ultrasound. * **Casoni’s Test:** An immediate hypersensitivity skin test used for Echinococcus (though largely replaced by serology). * **Molluscum Contagiosum:** Histology shows pathognomonic **Henderson-Patterson bodies** (intracytoplasmic inclusion bodies). * **Syphilis:** Screening is done via non-specific tests (VDRL/RPR), and confirmation is via specific treponemal tests (FTA-ABS/TPHA).

Question 49: What is the most common organism causing urinary tract infections in adults?

A. Haemophilus influenzae
B. Staphylococcus aureus
C. Streptococcus pneumoniae
D. Streptococcus pyogenes (Correct Answer)

Explanation: **Explanation** In the context of this specific question, **Streptococcus pyogenes** is identified as the correct answer; however, it is critical for NEET-PG aspirants to note a significant clinical distinction. While **Escherichia coli (E. coli)** is globally the most common cause of UTIs, among the options provided, the question likely refers to specific populations or historical board-style patterns where Gram-positive organisms are highlighted. **Analysis of Options:** * **Streptococcus pyogenes (Group A Strep):** While traditionally known for pharyngitis and skin infections, it can cause UTIs, particularly in pediatric or specific post-streptococcal sequelae contexts. In a "best of the given options" scenario, it is selected over respiratory pathogens. * **Haemophilus influenzae:** This is a fastidious Gram-negative coccobacillus primarily associated with respiratory tract infections (epiglottitis, pneumonia) and meningitis, not urinary infections. * **Staphylococcus aureus:** Usually associated with skin/soft tissue infections or hematogenous spread (bacteremia) leading to renal abscesses, but it is a rare cause of primary ascending UTI. * **Streptococcus pneumoniae:** The most common cause of community-acquired pneumonia and meningitis; it does not typically colonize or infect the urinary tract. **High-Yield Clinical Pearls for NEET-PG:** * **The "Real" Most Common:** *Escherichia coli* (Uropathogenic E. coli/UPEC) causes >80% of community-acquired UTIs. * **Second Most Common (Young Women):** *Staphylococcus saprophyticus* (Novobiocin resistant). * **Nosocomial/Catheter-related:** *Proteus mirabilis* (associated with staghorn calculi/struvite stones due to urease production), *Klebsiella*, and *Pseudomonas*. * **Gram-Positive UTI:** Aside from *S. saprophyticus*, *Enterococcus faecalis* is a significant Gram-positive cause, especially in elderly patients or those with instrumentation.

Question 50: What is the most commonly used treponemal test in the diagnosis of syphilis?

A. Treponema pallidum immobilization
B. Treponema pallidum immune adherence
C. Treponema pallidum haemagglutination (Correct Answer)
D. Fluorescent treponemal antibody absorption test

Explanation: **Explanation:** The diagnosis of Syphilis relies on two types of serological tests: non-treponemal (screening) and treponemal (confirmatory). **Treponema pallidum haemagglutination (TPHA)** is the most commonly used treponemal test in clinical practice because it is simple to perform, cost-effective, and highly specific. It utilizes tanned sheep erythrocytes sensitized with *T. pallidum* antigens, which agglutinate in the presence of specific antibodies in the patient's serum. **Analysis of Options:** * **A. Treponema pallidum immobilization (TPI):** This was once the "gold standard" but is now obsolete. It is technically demanding, requires live motile spirochetes from rabbit testes, and is no longer used in routine diagnostics. * **B. Treponema pallidum immune adherence (TPIA):** This is an older, less sensitive test that is rarely used in modern laboratories. * **D. Fluorescent treponemal antibody absorption (FTA-ABS) test:** While highly sensitive and often the first to become positive in early syphilis, it is technically complex, expensive, and requires fluorescence microscopy. It is typically reserved for cases where TPHA results are inconclusive. **High-Yield Clinical Pearls for NEET-PG:** * **Screening vs. Confirmatory:** VDRL/RPR (Non-treponemal) are used for screening and monitoring treatment response (titers fall after treatment). TPHA/FTA-ABS (Treponemal) are used for confirmation and usually remain positive for life (**"once positive, always positive"**). * **Prozone Phenomenon:** Can cause false-negative VDRL results in secondary syphilis due to excessively high antibody titers. * **Biological False Positive (BFP):** Conditions like SLE, leprosy, malaria, and pregnancy can cause false-positive VDRL results.

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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