Infectious Diseases Practice Questions

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

Tuberculosis. **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.

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

E. coli. **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.

Q: The Widal test is a type of which immunological test?

Agglutination test. **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.

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

The result is typically seen after 3 weeks.. 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.

Q: What is injected in the Mantoux test for tuberculosis?

Antigen abstract. ### 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).

Q: Which of the following are infections transmitted by blood?

All of the above. **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.

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

Pneumocystis pneumonia. **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 35 mmHg.

Q: Which infection is common after organ transplantation?

Cytomegalovirus. **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.

Question 1

Which of the following conditions is diagnosed using the Mantoux test?

Accepted Answer

Tuberculosis

Answer

Syphilis

Answer

Sarcoidosis

Answer

Hepatitis B

Question 2

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

Accepted Answer

E. coli

Answer

Rotavirus

Answer

Shigella

Answer

Ascaris

Question 3

The Widal test is a type of which immunological test?

Accepted Answer

Agglutination test

Answer

Flocculation test

Answer

Both

Answer

None

Question 4

Which of the following is NOT true about the Mantoux test?

Accepted Answer

The result is typically seen after 3 weeks.

Answer

PPD RT-23 with Tween 80 is used as the strain.

Answer

An induration of > 9mm is considered positive.

Answer

It is an example of Type IV Hypersensitivity.

Question 5

What is injected in the Mantoux test for tuberculosis?

Accepted Answer

Antigen abstract

Answer

Live attenuated antigen abstract

Answer

Killed bacteria

Answer

Live bacteria in low concentration

Question 6

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

Accepted Answer

Second Generation QuantiFERON-TB (Gold) assay used ESAT-6 and CPF-10.

Answer

First Generation QuantiFERON-TB assay used ESAT-6.

Answer

These tests can distinguish between M. tuberculosis and M. bovis.

Answer

None of the non-tuberculosis mycobacteria give a positive reaction with the test.

Question 7

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

Accepted Answer

Chlamydia

Answer

Klebsiella

Answer

E. coli

Answer

Mycoplasma

Question 8

Which of the following are infections transmitted by blood?

Accepted Answer

All of the above

Answer

HIV

Answer

Malaria

Answer

Toxoplasma

Question 9

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

Accepted Answer

Pneumocystis pneumonia

Answer

Antibodies to Human T-lymphotropic virus type II (HTLV-II)

Answer

Elevation of inducer/helper T cells

Answer

All of the above

Question 10

Which infection is common after organ transplantation?

Accepted Answer

Cytomegalovirus

Answer

Pneumocystis

Answer

Nocardia

Answer

Toxoplasma

Infectious Diseases — MCQs

Infectious Diseases — MCQs

On this page

Practice by Chapter

Want unlimited practice?