Infectious Diseases — MCQs

Infectious Diseases Indian Medical PG Practice Questions and MCQs

Question 21: Which of the following conditions is diagnosed using the Mantoux test?

A. Tuberculosis (Correct Answer)
B. Syphilis
C. Sarcoidosis
D. Hepatitis B

Explanation: **Explanation:** The **Mantoux test** (Tuberculin Skin Test) is the standard screening tool for **Tuberculosis (TB)**. It is based on a **Type IV (Delayed-type) Hypersensitivity reaction**. 1. **Why Tuberculosis is Correct:** The test involves the intradermal injection of 0.1 ml of **Purified Protein Derivative (PPD)** containing 5 Tuberculin Units (TU). If a person has been previously exposed to *Mycobacterium tuberculosis*, sensitized T-cells migrate to the injection site, releasing lymphokines that cause induration (hardness) within **48–72 hours**. A positive result indicates infection (latent or active) but does not distinguish between the two. 2. **Why Other Options are Incorrect:** * **Syphilis:** Diagnosed via microscopy (Dark-field) or serology (Non-specific: VDRL/RPR; Specific: TPHA/FTA-ABS). * **Sarcoidosis:** Historically associated with the **Kveim-Siltzbach test**, though diagnosis now relies on imaging and biopsy showing non-caseating granulomas. Interestingly, sarcoidosis often causes "anergy," leading to a *false-negative* Mantoux test. * **Hepatitis B:** Diagnosed using serological markers (HBsAg, Anti-HBs, HBeAg) and molecular methods (HBV-DNA PCR). **High-Yield Clinical Pearls for NEET-PG:** * **Injection Technique:** Must be intradermal (using a Tuberculin syringe) to create a "wheal." * **Interpretation:** Measure the **diameter of induration**, not erythema (redness). * **False Positive:** Seen in individuals vaccinated with **BCG** or infected with atypical mycobacteria. * **False Negative:** Seen in severe malnutrition, miliary TB, HIV/AIDS (low CD4 count), and immunosuppressive therapy. * **Two-step testing:** Used to detect the "Booster Effect" in healthcare workers.

Question 22: Which of the following is most commonly responsible for traveler's diarrhea?

A. E. coli (Correct Answer)
B. Rotavirus
C. Shigella
D. Ascaris

Explanation: **Explanation:** **Traveler’s Diarrhea (TD)** is defined as the passage of three or more unformed stools in a 24-hour period, often accompanied by abdominal cramps, nausea, or fever, occurring in individuals traveling from developed to developing regions. **Why E. coli is correct:** Enterotoxigenic *Escherichia coli* (**ETEC**) is the **most common cause** of traveler’s diarrhea worldwide (responsible for 30–50% of cases). It colonizes the small intestine and produces two types of toxins: **Heat-labile (LT)**, which increases cAMP, and **Heat-stable (ST)**, which increases cGMP. Both lead to the hypersecretion of water and electrolytes, resulting in watery diarrhea. **Why other options are incorrect:** * **Rotavirus:** While a leading cause of severe diarrhea in **infants and young children** globally, it is less common than ETEC in adult travelers. * **Shigella:** This is a cause of **inflammatory diarrhea (dysentery)** characterized by blood and mucus. While it can cause TD, it is significantly less frequent than ETEC. * **Ascaris:** *Ascaris lumbricoides* is a helminthic infection that typically presents with intestinal obstruction or nutritional deficiencies rather than acute, watery traveler's diarrhea. **High-Yield Clinical Pearls for NEET-PG:** * **Most common bacterial cause:** ETEC. * **Most common protozoal cause:** *Giardia lamblia* (often presents with foul-smelling, greasy stools and a longer incubation period). * **Prophylaxis:** Routine antibiotic prophylaxis is not recommended; however, **Bismuth subsalicylate** can be used. * **Treatment:** Rehydration is mainstay. For severe cases, **Fluoroquinolones** (like Ciprofloxacin) or **Azithromycin** (preferred in Southeast Asia due to resistance) are used.

Question 23: The Widal test is a type of which immunological test?

A. Flocculation test
B. Agglutination test (Correct Answer)
C. Both
D. None

Explanation: **Explanation:** The **Widal test** is a classic serological test used for the diagnosis of enteric fever (Typhoid and Paratyphoid). It is a type of **direct tube agglutination test** that detects specific antibodies (agglutinins) in the patient’s serum against the H (flagellar) and O (somatic) antigens of *Salmonella Typhi* and *Salmonella Paratyphi*. **1. Why Agglutination?** Agglutination occurs when a particulate antigen (like whole bacterial cells) reacts with its specific antibody, resulting in visible clumping. In the Widal test, the antigens used are killed bacterial suspensions. When antibodies are present in the patient's serum, they cross-link the bacteria, forming visible aggregates at the bottom of the test tube. **2. Analysis of Incorrect Options:** * **Flocculation test:** This is a variation of the precipitation test where the antigen is soluble, but the resulting precipitate remains suspended as "flakes" rather than settling. Examples include the **VDRL test** and **RPR** for Syphilis. * **Both/None:** These are incorrect because the Widal test specifically utilizes particulate antigens, categorizing it strictly under agglutination. **High-Yield Clinical Pearls for NEET-PG:** * **O Antigen:** Appears early, disappears early; indicates **recent/acute infection**. * **H Antigen:** Appears late, persists longer; indicates **convalescence** or past immunization. * **Diagnostic Titer:** In endemic areas like India, a titer of **>1:80 for O** and **>1:160 for H** is usually considered significant. * **Timing:** The test usually becomes positive during the **second week** of fever. * **False Positives:** Can occur in patients with chronic liver disease or those who have received the TAB vaccine.

Question 24: Which of the following is NOT true about the Mantoux test?

A. PPD RT-23 with Tween 80 is used as the strain.
B. The result is typically seen after 3 weeks. (Correct Answer)
C. An induration of > 9mm is considered positive.
D. It is an example of Type IV Hypersensitivity.

Explanation: The Mantoux test (Tuberculin Skin Test) is a screening tool for *Mycobacterium tuberculosis* infection. The correct answer is **B** because the statement is factually incorrect regarding the timing of the reaction. ### **Explanation of Options** * **Option B (Correct Answer):** The Mantoux test is a delayed-type hypersensitivity reaction. The result must be read **48 to 72 hours** (2–3 days) after the injection, not 3 weeks. If read after 72 hours, the result may be inaccurate due to the waning of the inflammatory response. * **Option A:** In India and many other regions, **PPD RT-23 (Purified Protein Derivative)** with **Tween 80** (a stabilizing detergent) is the standard reagent used. The standard dose is 2 Tuberculin Units (TU) in 0.1 ml. * **Option C:** In endemic areas like India, an induration (palpable raised hardening) of **10 mm or more** is generally considered positive. Therefore, a measurement of >9 mm is a standard threshold for a positive result in the general population. * **Option D:** The test is a classic clinical example of **Type IV (Delayed) Hypersensitivity**, mediated by T-lymphocytes (specifically Th1 cells) rather than antibodies. ### **High-Yield Clinical Pearls for NEET-PG** * **Injection Technique:** It is administered **intradermally** on the volar aspect of the forearm using a 26-gauge needle. * **Measurement:** Only the **induration** (the hard bump) is measured, not the erythema (redness). * **False Negatives:** Can occur in miliary TB, malnutrition, HIV/AIDS (low CD4 count), or recent viral infections (e.g., Measles). * **False Positives:** Can occur due to prior **BCG vaccination** or infection with Non-Tuberculous Mycobacteria (NTM). * **Booster Effect:** A second test may be positive if the first "primed" a dormant immune system; this is not a new infection.

Question 25: What is injected in the Mantoux test for tuberculosis?

A. Live attenuated antigen abstract
B. Killed bacteria
C. Antigen abstract (Correct Answer)
D. Live bacteria in low concentration

Explanation: ### Explanation The **Mantoux test** (Tuberculin Skin Test) is a diagnostic tool used to detect latent tuberculosis infection. It relies on a **Type IV (Delayed-type) Hypersensitivity reaction**. **Why "Antigen abstract" is correct:** The substance injected is **PPD (Purified Protein Derivative)**. It is an "antigen abstract" because it consists of specific soluble protein fractions extracted and purified from cultures of *Mycobacterium tuberculosis*. It does not contain the whole organism, but rather the antigenic components capable of stimulating T-cells in a sensitized individual. **Analysis of Incorrect Options:** * **A & D (Live attenuated/Live bacteria):** Injecting live *M. tuberculosis* would be dangerous and could cause disease. Live attenuated bacteria (BCG strain) are used for vaccination, not for skin testing. * **B (Killed bacteria):** While killed bacteria (like the Lepromin test) can elicit immune responses, the Mantoux test specifically uses purified protein extracts to standardize the dose (measured in Tuberculin Units) and reduce non-specific inflammatory reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Procedure:** 0.1 mL of PPD (containing 5 TU) is injected **intradermally** on the volar aspect of the forearm using a 26/27-gauge needle. * **Reading:** Results are interpreted after **48–72 hours** by measuring the diameter of **induration** (palpable raised hardening), not erythema (redness). * **False Negatives:** Can occur in miliary TB, malnutrition, AIDS (low CD4 count), and recent viral infections (e.g., Measles). This state is called **Anergy**. * **False Positives:** Common in individuals vaccinated with **BCG** or those infected with Non-Tuberculous Mycobacteria (NTM).

Question 26: Which of the following statements regarding Gamma-Release Assays for the diagnosis of Tuberculosis is true?

A. First Generation QuantiFERON-TB assay used ESAT-6.
B. Second Generation QuantiFERON-TB (Gold) assay used ESAT-6 and CPF-10. (Correct Answer)
C. These tests can distinguish between M. tuberculosis and M. bovis.
D. None of the non-tuberculosis mycobacteria give a positive reaction with the test.

Explanation: Interferon-Gamma Release Assays (IGRAs) are *in vitro* blood tests used to identify *Mycobacterium tuberculosis* (MTB) infection by measuring the T-cell release of interferon-gamma in response to specific antigens. ### **Explanation of Options** * **Correct Answer (B):** The **Second Generation QuantiFERON-TB (Gold)** assay improved specificity by using two highly specific recombinant antigens: **ESAT-6** (Early Secretory Antigenic Target-6) and **CFP-10** (Culture Filtrate Protein-10). These antigens are encoded by the RD1 (Region of Difference 1) segment of the MTB genome. * **Option A is incorrect:** The First Generation QuantiFERON-TB assay used **PPD (Purified Protein Derivative)** as the stimulant, which led to cross-reactivity with the BCG vaccine. * **Option C is incorrect:** IGRAs **cannot** distinguish between *M. tuberculosis* and *M. bovis* because both species contain the RD1 segment and express ESAT-6 and CFP-10. * **Option D is incorrect:** While IGRAs do not cross-react with most Non-Tuberculous Mycobacteria (NTM), a few species like ***M. kansasii, M. szulgai,*** and ***M. marinum*** also possess the RD1 segment and can cause a false-positive result. ### **High-Yield Clinical Pearls for NEET-PG** 1. **BCG Advantage:** Unlike the Tuberculin Skin Test (TST/Mantoux), IGRAs are **not affected by prior BCG vaccination**, making them superior for latent TB screening in vaccinated populations. 2. **Latent vs. Active:** IGRAs **cannot** differentiate between Latent TB Infection (LTBI) and Active TB disease. 3. **Third vs. Fourth Gen:** The **QFT-Plus** (4th Gen) added a second TB antigen tube to specifically stimulate CD8+ T-cells, enhancing sensitivity in immunocompromised patients. 4. **T-SPOT.TB:** This is the other major IGRA format which uses ELISPOT technology to count individual interferon-gamma secreting T-cells.

Question 27: Which organism is implicated as a causative factor in coronary artery disease?

A. Chlamydia (Correct Answer)
B. Klebsiella
C. E. coli
D. Mycoplasma

Explanation: **Explanation:** The association between infectious agents and atherosclerosis is a well-studied concept in cardiovascular pathology. Among the options provided, **Chlamydia pneumoniae** is the organism most strongly implicated in the pathogenesis of coronary artery disease (CAD). **1. Why Chlamydia is Correct:** * **Mechanism:** *C. pneumoniae* is an intracellular pathogen that can infect vascular endothelial cells, smooth muscle cells, and macrophages (foam cells) within arterial walls. * **Pathogenesis:** Chronic infection leads to persistent inflammation, which triggers the release of cytokines and promotes the formation and destabilization of atherosclerotic plaques. * **Evidence:** The organism has been identified within human atherosclerotic lesions via electron microscopy, PCR, and immunohistochemistry. Elevated serological titers of anti-Chlamydial antibodies are often correlated with an increased risk of myocardial infarction. **2. Why Other Options are Incorrect:** * **Klebsiella & E. coli:** These are primarily Gram-negative coliforms responsible for urinary tract infections, pneumonia, and sepsis. While they cause systemic inflammation, they do not have a proven direct role in the chronic inflammatory process of coronary atherogenesis. * **Mycoplasma:** While *Mycoplasma pneumoniae* causes atypical pneumonia and has been occasionally detected in plaques, the epidemiological and pathological evidence linking it to CAD is significantly weaker than that for *C. pneumoniae*. **High-Yield Clinical Pearls for NEET-PG:** * **Other implicated agents:** Besides *C. pneumoniae*, **Cytomegalovirus (CMV)** and **Helicobacter pylori** have also been studied for potential links to CAD. * **Diagnosis:** *C. pneumoniae* is best diagnosed via Microimmunofluorescence (MIF) tests or PCR. * **Treatment Note:** Despite the association, large-scale clinical trials (like the ACES and WIZARD trials) have shown that long-term antibiotic therapy (e.g., Azithromycin) does not significantly reduce the risk of secondary cardiovascular events.

Question 28: Which of the following are infections transmitted by blood?

A. HIV
B. Malaria
C. Toxoplasma
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (All of the above)**. Blood-borne transmission occurs when infectious pathogens (viruses, bacteria, or parasites) are spread through contact with infected blood, most commonly via blood transfusions, needle-stick injuries, or shared syringes. 1. **HIV (Human Immunodeficiency Virus):** This is a classic blood-borne virus. It resides in high titers in the blood and is transmitted through parenteral routes, organ transplantation, and vertical transmission (mother-to-child). 2. **Malaria:** Caused by *Plasmodium* species, this is an intra-erythrocytic parasite. While primarily transmitted by the female *Anopheles* mosquito, it can be transmitted via blood transfusion (**Transfusion Malaria**) or contaminated needles. Notably, transfusion malaria has a shorter incubation period because it bypasses the pre-erythrocytic (hepatic) phase. 3. **Toxoplasma gondii:** While most commonly acquired through the ingestion of oocysts (cat feces) or tissue cysts (undercooked meat), *Toxoplasma* can be transmitted via blood transfusion and organ transplantation, especially in immunocompromised recipients. **Clinical Pearls for NEET-PG:** * **Screening:** In India, mandatory screening of blood donors includes HIV 1 & 2, Hepatitis B (HBsAg), Hepatitis C (HCV), Syphilis, and Malaria. * **Window Period:** The time between infection and laboratory detectability. Nucleic Acid Testing (NAT) is used to reduce this period for HIV and HCV. * **Other Blood-borne Pathogens:** Hepatitis B, Hepatitis C, Cytomegalovirus (CMV), and HTLV-1/2. * **Prion Disease:** Creutzfeldt-Jakob Disease (CJD) is also a theoretical risk in blood transfusions.

Question 29: Homosexual men with AIDS are most likely to exhibit which of the following conditions?

A. Antibodies to Human T-lymphotropic virus type II (HTLV-II)
B. Pneumocystis pneumonia (Correct Answer)
C. Elevation of inducer/helper T cells
D. All of the above

Explanation: **Explanation:** **Pneumocystis pneumonia (PCP)**, caused by the fungus *Pneumocystis jirovecii*, is the most common opportunistic infection in patients with AIDS. In the context of the NEET-PG, it is crucial to remember that while the epidemiology of HIV has evolved, PCP remains a hallmark AIDS-defining illness, particularly in patients with CD4+ T-cell counts below 200 cells/mm³. **Analysis of Options:** * **Option B (Correct):** *Pneumocystis jirovecii* is an opportunistic pathogen that specifically exploits the cell-mediated immunodeficiency seen in AIDS. Homosexual men (MSM) were among the first cohorts identified with this association, and it remains a leading cause of morbidity and mortality in untreated HIV. * **Option A (Incorrect):** HTLV-II is primarily associated with intravenous drug users (IDUs) rather than homosexual men. While co-infections occur, it is not the "most likely" condition compared to PCP. * **Option C (Incorrect):** HIV selectively infects and destroys CD4+ (inducer/helper) T cells. Therefore, AIDS is characterized by a **profound depletion** (decrease) of these cells, leading to an inversion of the normal CD4:CD8 ratio. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** Start Trimethoprim-Sulfamethoxazole (TMP-SMX) when CD4 count < 200 cells/mm³. * **Diagnosis:** Silver stains (Gomori Methenamine Silver - GMS) or Direct Fluorescent Antibody (DFA) testing of induced sputum or bronchoalveolar lavage (BAL) showing "crushed ping-pong ball" shaped cysts. * **Radiology:** Classic "ground-glass opacities" (GGO) on HRCT and bilateral perihilar infiltrates on X-ray. * **Treatment:** High-dose TMP-SMX is the drug of choice; add steroids if PaO₂ < 70 mmHg or A-a gradient > 35 mmHg.

Question 30: Which infection is common after organ transplantation?

A. Pneumocystis
B. Nocardia
C. Cytomegalovirus (Correct Answer)
D. Toxoplasma

Explanation: **Explanation:** **Cytomegalovirus (CMV)** is the most common clinically significant viral infection following solid organ transplantation (SOT). The underlying medical concept is the **reactivation of latent virus** (or primary infection from the donor) during the period of maximal immunosuppression, typically occurring **1 to 6 months post-transplant**. CMV is a member of the Herpesviridae family and remains latent in myeloid progenitor cells and monocytes. It is a major cause of morbidity, leading to "CMV syndrome" (fever, malaise, cytopenia) or tissue-invasive disease (pneumonitis, hepatitis, colitis). **Analysis of Incorrect Options:** * **Pneumocystis jirovecii (A):** While a significant opportunistic fungal infection causing pneumonia in immunocompromised hosts, its incidence has drastically decreased due to the routine use of **TMP-SMX prophylaxis** in transplant recipients. * **Nocardia (B):** This is an opportunistic bacterial infection (Gram-positive, weakly acid-fast) that can cause pulmonary or CNS disease, but it is far less frequent than CMV. * **Toxoplasma gondii (D):** This protozoan infection is primarily a concern in heart transplant recipients (if the donor is seropositive) or HIV patients, but it does not match the overall prevalence of CMV across all organ types. **High-Yield Clinical Pearls for NEET-PG:** * **Timeline:** CMV typically appears in the "middle period" (1–6 months). Infections in the first month are usually bacterial/surgical; after 6 months, they are community-acquired. * **Diagnosis:** **PP65 antigenemia assay** or **Quantitative PCR** for CMV DNA. * **Histology:** Look for **"Owl’s eye" intranuclear inclusions**. * **Treatment:** **Ganciclovir** or Valganciclovir are the drugs of choice.

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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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