Infectious Diseases Practice Questions

Q: What is the most common site where organisms are present in a typhoid carrier?

Gall Bladder. **Explanation:** The correct answer is **Gall Bladder**. **Why Gall Bladder is correct:** *Salmonella Typhi* is a Gram-negative, facultative intracellular bacillus. In chronic carriers (individuals who shed the bacilli in stools for >1 year after the initial attack), the organisms persist in the biliary tract. The **Gall Bladder** is the most common site of colonization, particularly in patients with pre-existing gallstones or cholecystitis. The bacteria survive within the bile and form biofilms on the surface of gallstones, which protects them from antibiotics and the host immune system. From the gall bladder, the organisms are intermittently discharged into the intestine and excreted in the feces. **Why other options are incorrect:** * **Spleen & Liver:** While these are part of the Reticuloendothelial System (RES) where *S. Typhi* multiplies during the incubation period and the first week of illness (causing hepatosplenomegaly), they are not the primary sites for long-term chronic carriage. * **Salivary gland:** This is not a reservoir for *S. Typhi*. Salivary glands are more commonly associated with viral infections like Mumps or bacterial infections like *Staphylococcus aureus*. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Carrier Definition:** Shedding of *S. Typhi* in feces/urine for more than **12 months**. * **Urinary Carriers:** Less common than fecal carriers; usually associated with urinary tract abnormalities like **Schistosomiasis** or kidney stones. * **Famous Case:** "Typhoid Mary" (Mary Mallon) was a classic fecal carrier. * **Diagnosis of Carrier State:** Best detected by **repeated stool cultures** or the **Vi agglutination test** (screening for Vi antibodies). * **Treatment:** The drug of choice for eradicating the carrier state is **Ciprofloxacin** (for 4–6 weeks). If gallstones are present, cholecystectomy may be required.

Q: What is the most common opportunistic infection in AIDS patients?

Tuberculosis. **Explanation:** In the context of the HIV/AIDS pandemic, **Tuberculosis (TB)**, caused by *Mycobacterium tuberculosis*, is the most common opportunistic infection (OI) and the leading cause of mortality worldwide. While HIV-infected individuals are susceptible to a wide range of pathogens, TB is unique because it can occur at any CD4 T-cell count, although the risk increases significantly as the count drops. In India and other developing nations, TB remains the most frequent co-infection encountered in clinical practice. **Analysis of Options:** * **Tuberculosis (A):** Correct. It is the most common OI globally and in India. It often presents atypically in advanced AIDS (extrapulmonary or disseminated forms). * **Pneumocystis jirovecii pneumonia (C):** This was historically the most common opportunistic infection in the Western world before the widespread use of prophylactic Co-trimoxazole. It typically occurs when the CD4 count falls below 200 cells/µL. * **Cryptococcosis (B):** This is the most common fungal meningitis in AIDS patients, usually occurring at CD4 counts <100 cells/µL, but it is less frequent than TB. * **Histoplasmosis (D):** This is a regional fungal infection (endemic in specific river valleys) and is not as globally prevalent as TB or PCP. **High-Yield Clinical Pearls for NEET-PG:** * **Most common OI overall:** Tuberculosis. * **Most common fungal OI:** Candidiasis (Oral Thrush). * **Most common CNS mass lesion:** Toxoplasmosis. * **Most common cause of blindness:** CMV Retinitis (CD4 <50 cells/µL). * **Screening:** All HIV-positive patients should be screened for TB using the four-symptom complex (cough, fever, night sweats, and weight loss).

Q: All of the following are true regarding Lyme disease except?

Polymorphonuclear lymphocytosis in CSF suggests meningial involvement.. **Explanation:** Lyme disease, caused by the spirochete *Borrelia burgdorferi*, presents in three stages: early localized, early disseminated, and late persistent. **Why Option C is the correct (False) statement:** In Lyme meningitis (Stage 2), the characteristic finding in the Cerebrospinal Fluid (CSF) is **lymphocytic pleocytosis** (an increase in lymphocytes), not polymorphonuclear (neutrophilic) lymphocytosis. Neutrophilic predominance is typically seen in acute bacterial meningitis, whereas spirochetal infections like Lyme and Syphilis characteristically cause a mononuclear/lymphocytic response. **Analysis of other options:** * **Option A:** True. After a tick bite (*Ixodes*), the spirochetes replicate locally to produce the classic **Erythema Migrans** (bull’s eye rash) and then invade locally through the dermis before hematogenous spread. * **Option B:** True. Despite a strong humoral (antibody) response, the infection can persist. *Borrelia* employs **antigenic variation** (VlsE surface protein) to evade the host immune system, allowing it to survive in joints and the nervous system. * **Option C:** False. (As explained above). * **Option D:** True. The detection of **intrathecal antibody production** (specifically IgM or IgA) against *B. burgdorferi* antigens is a gold-standard diagnostic marker for Neuroborreliosis, confirming that the immune response is occurring within the CNS. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes* tick (deer tick). * **Stage 1:** Erythema Migrans (pathognomonic). * **Stage 2:** Bilateral Facial Nerve (CN VII) palsy is a classic board-favorite presentation. * **Stage 3:** Chronic large joint arthritis (usually the knee). * **Treatment:** Doxycycline is the drug of choice; Ceftriaxone is used for neurological or cardiac manifestations.

Q: Freshwater swimming can lead to which of the following infections?

Leptospirosis. **Explanation:** **Leptospirosis** is a zoonotic infection caused by the spirochete *Leptospira interrogans*. The primary reservoir is rodents (rats), which shed the bacteria in their urine. Humans acquire the infection through direct or indirect contact with water or soil contaminated by this infected urine. Freshwater swimming, white-water rafting, and occupational exposure (sewer workers, farmers) are classic risk factors because the bacteria enter the body through skin abrasions or mucous membranes. **Analysis of Incorrect Options:** * **Brucellosis:** Primarily transmitted through the consumption of unpasteurized dairy products or direct contact with infected livestock (cattle, goats). It is not associated with water activities. * **Babesiosis:** A malaria-like parasitic disease transmitted by the bite of the *Ixodes* tick. It is a vector-borne disease, not waterborne. * **Lassa Fever:** A viral hemorrhagic fever caused by an Arenavirus. It is transmitted through contact with food or household items contaminated with the excreta of infected *Mastomys* rats, mainly in West Africa. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Ranges from a mild flu-like illness to **Weil’s Disease**, characterized by the triad of **jaundice, renal failure, and hemorrhage**. * **Conjunctival Suffusion:** A pathognomonic sign (redness of eyes without inflammatory exudate). * **Diagnosis:** **MAT (Microscopic Agglutination Test)** is the gold standard. * **Drug of Choice:** Doxycycline (prophylaxis/mild cases) or IV Penicillin G (severe cases).

Q: The Tuberculin test is administered via which route of administration?

Intradermal. The Tuberculin Skin Test (TST), also known as the **Mantoux test**, is a classic example of a **Type IV (Delayed-type) Hypersensitivity reaction**. ### Why Intradermal is Correct The test involves the injection of 0.1 mL of **Purified Protein Derivative (PPD)** containing 5 Tuberculin Units (TU). The **intradermal route** is essential because the dermis contains a high concentration of antigen-presenting cells (Langerhans cells). These cells process the PPD and present it to sensitized T-lymphocytes. If a person has been previously exposed to *M. tuberculosis*, a localized inflammatory response occurs, resulting in **induration** (palpable hardness) within 48–72 hours. ### Why Other Options are Incorrect * **Subcutaneous:** Injecting PPD into the fatty tissue below the dermis leads to rapid absorption into the systemic circulation. This prevents the localized cellular interaction required for a visible skin reaction, leading to a **false-negative** result. * **Intramuscular:** Similar to the subcutaneous route, the vascularity of muscle tissue causes the antigen to be carried away too quickly to trigger a local delayed-type hypersensitivity response. * **Subdermal:** This is a non-standard medical term often confused with intradermal; however, the specific technique required for Mantoux is strictly intradermal (forming a 6–10 mm wheal). ### NEET-PG High-Yield Pearls * **Reading the Test:** Measure the diameter of **induration** (not erythema) transverse to the long axis of the forearm. * **False Positives:** Common in individuals vaccinated with **BCG** or those with Non-Tuberculous Mycobacteria (NTM) infections. * **False Negatives (Anergy):** Can occur in miliary TB, HIV/AIDS (low CD4 count), malnutrition, or recent viral infections (e.g., Measles). * **Alternative:** The **IGRA (Interferon-Gamma Release Assay)** is preferred for BCG-vaccinated individuals as it is more specific.

Q: Which of the following is NOT caused by a virus?

Rocky Mountain spotted fever. **Explanation:** The correct answer is **Rocky Mountain spotted fever (RMSF)** because it is a bacterial infection, not a viral one. **1. Why Option A is correct:** Rocky Mountain spotted fever is caused by ***Rickettsia rickettsii***, an obligate intracellular **Gram-negative bacterium**. It is transmitted to humans through the bite of infected ticks (e.g., *Dermacentor variabilis*). Unlike viruses, *Rickettsia* species have both DNA and RNA, possess a cell wall, and are susceptible to antibiotics like Doxycycline. **2. Why the other options are incorrect:** * **Kyasanur Forest Disease (KFD):** Caused by the KFD virus, a member of the family *Flaviviridae*. It is a tick-borne viral hemorrhagic fever endemic to Karnataka, India. * **Dengue Fever:** Caused by the Dengue virus (DENV 1-4), a mosquito-borne *Flavivirus* transmitted primarily by *Aedes aegypti*. * **Yellow Fever:** Caused by the Yellow fever virus, another member of the *Flaviviridae* family, also transmitted by *Aedes* mosquitoes. **3. NEET-PG Clinical Pearls:** * **Drug of Choice:** For almost all Rickettsial infections (including RMSF and Scrub Typhus), **Doxycycline** is the treatment of choice, regardless of the patient's age. * **The Triad:** The classic clinical triad of RMSF is fever, headache, and a characteristic petechial rash that typically begins on the wrists and ankles before spreading centrally. * **Vector Identification:** Remember that KFD is transmitted by the hard tick ***Haemaphysalis spinigera***, a frequent high-yield fact in Indian exams. * **Flavivirus Family:** Dengue, Yellow Fever, KFD, West Nile, and Zika all belong to the *Flaviviridae* family.

Q: A patient with HIV presents with diarrhea and acid-fast bacilli (AFB) positive in stool. What is the most likely causative organism?

Mycobacterium avium intracellulare. **Explanation:** The correct answer is **Mycobacterium avium intracellulare (MAC)**. In patients with advanced HIV/AIDS (typically with CD4 counts <50 cells/mm³), **Mycobacterium avium complex (MAC)** is the most common cause of disseminated opportunistic bacterial infection. While MAC primarily affects the reticuloendothelial system, it frequently involves the gastrointestinal tract, leading to chronic diarrhea, abdominal pain, and malabsorption. A key diagnostic feature is the presence of **Acid-Fast Bacilli (AFB)** in the stool or biopsy, representing the organism's acid-fast nature due to mycolic acid in its cell wall. **Why other options are incorrect:** * **Mycobacterium tuberculosis (MTB):** While MTB is common in HIV patients, it usually presents with pulmonary symptoms or ileocecal tuberculosis (causing constipation or obstruction rather than simple chronic diarrhea). It is less likely than MAC to show positive AFB in a stool sample unless there is active intestinal TB. * **Mycobacterium leprae:** This organism causes Leprosy, affecting the skin and peripheral nerves. It does not cause gastrointestinal disease or diarrhea. * **Mycoplasma:** These are the smallest free-living organisms and **lack a cell wall**; therefore, they are not acid-fast and cannot be detected via AFB staining. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** Azithromycin or Clarithromycin is indicated for MAC prophylaxis in HIV patients when CD4 <50 cells/mm³. * **Diagnosis:** Gold standard is blood culture (using BACTEC) or bone marrow culture. * **Stain:** MAC appears as short, coccobacillary acid-fast bacilli, often found within macrophages (foamy histiocytes). * **Differential Diagnosis:** If a stool AFB is positive in an HIV patient, also consider **Cryptosporidium parvum**, but these appear as acid-fast **oocysts**, not bacilli.

Question 1

All are true about Immune Reconstitution Inflammatory syndrome (IRIS) except?

Accepted Answer

It occurs when CD4 cell count is < 50 cells/mm3

Answer

Develops after initiation of anti-retroviral therapy

Answer

Associated with delayed type of hypersensitivity

Answer

Does not require a specific antimicrobial therapy for recovery

Question 2

For a neonate born to a mother with suspected chlamydial infection, which sample should be taken for diagnosis?

Accepted Answer

Conjunctival swab

Answer

Urethral swab

Answer

Urine sample

Answer

Blood sample

Question 3

What is the most common site where organisms are present in a typhoid carrier?

Accepted Answer

Gall Bladder

Answer

Spleen

Answer

Liver

Answer

Salivary gland

Question 4

What is the most common opportunistic infection in AIDS patients?

Accepted Answer

Tuberculosis

Answer

Cryptococcosis

Answer

Pneumocystis jirovecii pneumonia

Answer

Histoplasmosis

Question 5

Individuals with hypogammaglobulinemia are at particular risk of infectious diarrhea due to which of the following pathogens?

Accepted Answer

Clostridioides difficile and giardiasis

Answer

Giardiasis and shigellosis

Answer

Clostridioides difficile and Vibrio parahaemolyticus

Answer

Vibrio parahaemolyticus and shigellosis

Question 6

All of the following are true regarding Lyme disease except?

Accepted Answer

Polymorphonuclear lymphocytosis in CSF suggests meningial involvement.

Answer

Borrelia burgdorferi replicates locally and invades locally.

Answer

Infection progresses in spite of good humoral immunity.

Answer

IgA intrathecally confirms meningitis.

Question 7

Freshwater swimming can lead to which of the following infections?

Accepted Answer

Leptospirosis

Answer

Brucellosis

Answer

Babesiosis

Answer

Lassa fever

Question 8

The Tuberculin test is administered via which route of administration?

Accepted Answer

Intradermal

Answer

Subcutaneous

Answer

Intramuscular

Answer

Subdermal

Question 9

Which of the following is NOT caused by a virus?

Accepted Answer

Rocky Mountain spotted fever

Answer

Kyasanur Forest Disease (KFD)

Answer

Dengue fever

Answer

Yellow fever

Question 10

A patient with HIV presents with diarrhea and acid-fast bacilli (AFB) positive in stool. What is the most likely causative organism?

Accepted Answer

Mycobacterium avium intracellulare

Answer

Mycobacterium tuberculosis

Answer

Mycobacterium leprae

Answer

Mycoplasma

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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