Infectious Diseases Practice Questions

Q: Cerebrospinal fluid (CSF) adenosine deaminase levels are significantly higher in which of the following conditions?

Tuberculous meningitis. **Explanation:** **Adenosine Deaminase (ADA)** is an enzyme involved in purine metabolism, primarily produced by T-lymphocytes during their activation and proliferation. Its measurement in body fluids serves as a surrogate marker for cell-mediated immunity. **1. Why Tuberculous Meningitis (TBM) is correct:** In TBM, the body mounts a robust cell-mediated immune response [1]. The infiltration and activation of T-lymphocytes in the subarachnoid space lead to significantly elevated levels of ADA in the CSF [2]. A cut-off value of **>10 U/L** is highly suggestive of TBM, with high sensitivity and specificity, making it a crucial rapid diagnostic tool while waiting for culture results. **2. Why other options are incorrect:** * **Bacterial Meningitis:** While ADA can be slightly elevated due to neutrophils, the levels are typically much lower than in TBM [3]. The primary diagnostic markers here are low glucose, high protein, and a predominant neutrophilic pleocytosis. * **Viral Meningitis:** This condition usually presents with normal or only minimally elevated ADA levels because the lymphocytic response is not as intense as the granulomatous inflammation seen in TB [3]. * **Syphilitic Meningitis:** Although it causes a lymphocytic pleocytosis, it does not typically trigger the specific T-cell activation pathway that results in high ADA levels. **Clinical Pearls for NEET-PG:** * **Gold Standard for TBM:** CSF Culture (BACTEC/MGIT) or GeneXpert (NAAT). * **CSF Profile in TBM:** Fibrin web/cobweb formation, high protein, low glucose, and lymphocytic pleocytosis [3]. * **ADA in other fluids:** Elevated ADA is also highly diagnostic in **Pleural effusion** and **Peritoneal fluid** (Ascites) for Tuberculosis. * **False Positives:** CSF ADA can also be elevated in CNS Lymphoma and Neurobrucellosis.

Q: What is true about Rabies?

Bats transmit the disease in some parts of the world.. **Explanation:** **1. Why Option A is correct:** Rabies is a zoonotic viral disease caused by Lyssaviruses. While domestic dogs are the primary source of human rabies deaths globally (especially in Asia and Africa), **bats** are the most common source of rabies in the Americas and parts of Europe. In these regions, bat-mediated rabies is a significant public health concern, making this statement factually accurate. **2. Why the other options are incorrect:** * **Option B:** While rabies involves the brainstem, the characteristic pathology is **Limbic Encephalitis**. The virus primarily affects the hippocampus (Ammon’s horn) and the cerebellum (Purkinje cells), rather than being isolated to the brainstem. * **Option C:** Negri bodies (pathognomonic eosinophilic cytoplasmic inclusions) are most commonly found in the **Hippocampus (Ammon’s horn)** and the **Purkinje cells of the cerebellum**. They are not typically associated with the anterior pituitary. * **Option D:** **Paraesthesia** (tingling, itching, or pain) at the site of the bite is actually the **most important early diagnostic sign** of rabies, occurring in 50-80% of patients during the prodromal phase. **Clinical Pearls for NEET-PG:** * **Incubation Period:** Usually 1–3 months (highly variable based on the distance of the bite from the CNS). * **Hydrophobia:** Pathognomonic sign caused by forceful spasms of the diaphragm and accessory respiratory muscles when attempting to swallow. * **Diagnosis:** Intra-vitam diagnosis is made via **Skin biopsy** (from the nape of the neck) for viral antigen or **PCR** of saliva/CSF. * **Post-Exposure Prophylaxis (PEP):** Includes wound washing, Rabies Vaccine (Days 0, 3, 7, 14, 28), and Rabies Immunoglobulin (RIG) for Category III bites [1].

Q: A 37-year-old female presents to the emergency room with a fever. Chest x-ray shows multiple patchy infiltrates in both lungs. Echocardiography and blood cultures suggest a diagnosis of acute bacterial endocarditis limited to the tricuspid valve. Which of the following is the most probable etiology?

Illicit drug use. The clinical presentation of **fever, multiple patchy lung infiltrates, and tricuspid valve endocarditis** is a classic triad for **Right-Sided Infective Endocarditis (RSIE)**. **1. Why Illicit Drug Use is Correct:** Intravenous drug use (IVDU) is the most significant risk factor for right-sided endocarditis [2]. When non-sterile needles are used, bacteria (most commonly *Staphylococcus aureus*) enter the venous system and first encounter the tricuspid valve [3]. The "patchy infiltrates" on the chest X-ray represent **septic pulmonary emboli**, which occur when infected vegetations break off the tricuspid valve and travel through the pulmonary artery into the lungs [1]. **2. Why Other Options are Incorrect:** * **Congenital Heart Disease (CHD):** While CHD (e.g., VSD, PDA) predisposes patients to endocarditis, it typically affects the left side or specific pressure-gradient sites. It is a less common cause of isolated tricuspid involvement compared to IVDU. * **Rheumatic Fever:** Chronic rheumatic heart disease predominantly affects the **Mitral valve** (most common) followed by the Aortic valve. Isolated tricuspid involvement is extremely rare in rheumatic cases. * **Rheumatoid Arthritis:** While RA can cause sterile valvular nodules or pericarditis, it is not a recognized cause of acute bacterial endocarditis. **3. NEET-PG High-Yield Pearls:** * **Most common organism in IVDU:** *Staphylococcus aureus* (often MRSA) [3]. * **Most common valve involved in IVDU:** Tricuspid Valve. * **Most common valve involved overall (Non-IVDU):** Mitral Valve. * **Radiological Hallmark:** Multiple peripheral wedge-shaped opacities or cavitary lesions (septic emboli) [1]. * **Clinical Sign:** A systolic murmur that increases with inspiration (**Carvallo’s sign**) suggests tricuspid regurgitation [2].

Q: What is the principal vector of the dengue virus?

Aedes aegypti. **Explanation:** **Dengue virus (DENV)**, a flavivirus, is primarily transmitted to humans through the bite of infected female mosquitoes of the genus *Aedes*. **Aedes aegypti** is the principal vector because it is highly anthropophilic (prefers human blood), lives in close proximity to human habitations, and breeds in stagnant clean water (e.g., flower pots, discarded tires). While *Aedes albopictus* is a secondary vector, *A. aegypti* remains the most efficient transmitter due to its frequent feeding behavior. **Analysis of Incorrect Options:** * **Anopheles culicifacies:** This is the primary vector for **Malaria** in rural India. * **Anopheles stephensi:** This is the major vector for **Malaria** in urban areas of India. * **Culex molestus:** Species of the *Culex* genus are primarily responsible for transmitting **Japanese Encephalitis** and **Lymphatic Filariasis** (specifically *Culex quinquefasciatus*). **High-Yield Clinical Pearls for NEET-PG:** * **Feeding Pattern:** *Aedes* mosquitoes are "day biters," with peak activity during early morning and late afternoon. * **Nervous Feeding:** They are "interrupted feeders," meaning they bite multiple people to complete a single blood meal, leading to rapid outbreaks within a household. * **Transovarial Transmission:** The virus can pass from the female mosquito to her eggs, allowing the virus to persist even during dry seasons. * **Control:** The most effective prevention is source reduction (eliminating breeding sites) and using larvicides like **Abate (Temephos)**.

Q: What is true about miliary tuberculosis?

Commonly affects liver, kidney and spleen. **Explanation:** Miliary tuberculosis (TB) is a form of disseminated tuberculosis resulting from the **hematogenous (blood-borne) spread** of *Mycobacterium tuberculosis* [1]. **Why Option D is Correct:** When TB bacilli enter the bloodstream, they are filtered by the reticuloendothelial system. This leads to the formation of tiny, discrete foci of infection (1–2 mm granulomas resembling millet seeds) across multiple organs. The **liver, spleen, bone marrow, kidneys, and lungs** are the most frequently involved sites [1]. Clinical findings often include hepatosplenomegaly and lymphadenopathy [1]. **Analysis of Incorrect Options:** * **Options A, B, and C:** Miliary TB is not exclusive to a specific phase of the disease. It can occur during **primary infection** (early hematogenous spread before cell-mediated immunity is fully established) or as a result of **reactivation** of a latent focus (secondary TB) [1]. Because it can occur in both scenarios—and even as a result of direct iatrogenic vascular erosion—stating it occurs "following" only one specific type of reactivation is restrictive and technically inaccurate in this context. **High-Yield Clinical Pearls for NEET-PG:** * **Chest X-ray:** Shows a classic "millet seed" pattern (uniform 1–2 mm nodules) throughout both lung fields [1]. * **Choroid Tubercles:** Pathognomonic yellowish lesions on the retina (fundoscopy) are seen in ~30% of cases [1]. * **Diagnosis:** Gold standard is often a biopsy (liver or bone marrow) showing granulomas, or culture of body fluids. * **Auregnostic sign:** In elderly patients, "cryptic miliary TB" can present as a fever of unknown origin (FUO) without classic radiologic findings.

Q: A 60-year-old nursing home resident presents with a 3-day history of progressive shortness of breath and cough. The lung examination reveals right basilar crackles. The chest x-ray shows right lower lobe consolidation. Sputum culture grows methicillin-resistant Staphylococcus aureus (MRSA). Select the most appropriate isolation precaution.

Contact precautions. ### Explanation The correct answer is **Contact Precautions**. **1. Why Contact Precautions are correct:** Methicillin-resistant *Staphylococcus aureus* (MRSA) is primarily transmitted via direct or indirect contact with the patient or their environment (fomites) [1]. In healthcare settings, especially nursing homes and hospitals, MRSA can colonize the skin and respiratory tract. To prevent horizontal transmission between patients—often via the hands of healthcare workers—**Contact Precautions** (gloves and gown) are mandatory and maintenance of good hand hygiene is the most important prevention practice [1]. This applies to MRSA regardless of the site of infection (pneumonia, skin, or bloodstream). **2. Why other options are incorrect:** * **Standard Precautions (A):** These are used for all patients regardless of diagnosis. While they include hand hygiene, they are insufficient for multidrug-resistant organisms (MDROs) like MRSA, which require an additional layer of protection [1]. * **Droplet Precautions (C):** These are used for pathogens transmitted by large respiratory droplets (e.g., *N. meningitidis*, Influenza, Pertussis). While MRSA causes pneumonia, it is not typically spread through the air in large droplets that require a mask. * **Airborne Precautions (D):** These are reserved for pathogens transmitted via small-particle aerosols that remain suspended in the air (e.g., TB, Measles, Varicella). MRSA does not spread via this route. **3. NEET-PG Clinical Pearls:** * **MRSA Treatment:** Vancomycin is the drug of choice [2]. For MRSA pneumonia specifically, **Linezolid** is often preferred over Vancomycin due to better lung tissue penetration [2]. * **Daptomycin Warning:** Never use Daptomycin for MRSA pneumonia; it is inactivated by pulmonary surfactant. * **Decolonization:** For chronic MRSA carriers, intranasal **Mupirocin** and Chlorhexidine baths are used to eradicate the carrier state.

Q: A 6-year-old child presents with perianal pruritus, skin excoriation, and nocturnal enuresis, and is found to be infected with a parasite capable of autoinfection. What is the infective form of this parasite for humans?

Embryonated egg. ### Explanation **Diagnosis:** The clinical presentation of **perianal pruritus** (worse at night), skin excoriation, and **nocturnal enuresis** in a child is classic for **Enterobius vermicularis** (Pinworm/Seatworm). **1. Why the Correct Answer is Right:** The life cycle of *E. vermicularis* begins when the gravid female migrates to the perianal skin at night to deposit eggs. These eggs become **embryonated (infective)** within 4 to 6 hours. Infection occurs via the fecal-oral route when these embryonated eggs are ingested. The parasite is unique because it can cause **retro-infection** (larvae hatch on the perianal skin and migrate back into the colon) and **autoinfection** (hand-to-mouth transfer due to scratching), both of which involve the embryonated egg as the starting point of the cycle. **2. Why Incorrect Options are Wrong:** * **A. Filariform larvae:** This is the infective stage for *Strongyloides stercoralis*, *Ancylostoma duodenale*, and *Necator americanus* [1]. While *Strongyloides* also causes autoinfection, it presents with "larva currens" (transient itchy linear weals) and respiratory/GI symptoms rather than isolated perianal pruritus [1]. * **C & D. Adult female/male:** Adult worms live in the cecum and appendix. While the migrating female causes the symptoms, the adult stage is not the mode of transmission or the infective form for humans. **3. NEET-PG High-Yield Pearls:** * **Gold Standard Diagnosis:** NIH Swab or **Scotch Tape Test** (Cellophane tape test) performed early in the morning. Eggs are typically not found in routine stool exams. * **Morphology:** Eggs are characteristic **"D-shaped"** (planoconvex) and possess a double-layered shell. * **Treatment:** Albendazole or Mebendazole (single dose, repeated after 2 weeks). **Treat the entire family** to prevent reinfection. * **Associated Condition:** Pinworms are a known (though rare) cause of acute appendicitis in children.

Question 1

Which of the following is the LEAST common immunologic manifestation of HIV infection?

Accepted Answer

Anaphylactic reactions to drugs

Answer

Cutaneous reactions to drugs

Answer

Anticardiolipin antibodies

Answer

Oligoarticular arthritis

Question 2

Cerebrospinal fluid (CSF) adenosine deaminase levels are significantly higher in which of the following conditions?

Accepted Answer

Tuberculous meningitis

Answer

Bacterial meningitis

Answer

Viral meningitis

Answer

Syphilitic meningitis

Question 3

A patient with diarrhea has a fecal smear that is negative for leukocytes. The patient's diarrhea is most likely caused by

Accepted Answer

Enterotoxigenic Escherichia coli

Answer

Campylobacter enteritis

Answer

Shigellosis

Answer

Typhoid fever

Question 4

What is true about Rabies?

Accepted Answer

Bats transmit the disease in some parts of the world.

Answer

Brainstem encephalitis is characteristic.

Answer

Negri bodies are commonly found in the thalamus and anterior pituitary.

Answer

Paraesthesia is not seen.

Question 5

A 37-year-old female presents to the emergency room with a fever. Chest x-ray shows multiple patchy infiltrates in both lungs. Echocardiography and blood cultures suggest a diagnosis of acute bacterial endocarditis limited to the tricuspid valve. Which of the following is the most probable etiology?

Accepted Answer

Illicit drug use

Answer

Congenital heart disease

Answer

Rheumatic fever

Answer

Rheumatoid arthritis

Question 6

Which of the following is the classical CSF finding seen in tuberculous meningitis (TBM)?

Accepted Answer

Increased protein, decreased sugar, increased lymphocytes

Answer

Increased protein, sugar and lymphocytes

Answer

Decreased protein, increased sugar and lymphocytes

Answer

Increased sugar, protein and neutrophils

Question 7

What is the principal vector of the dengue virus?

Accepted Answer

Aedes aegypti

Answer

Anopheles culicifacies

Answer

Anopheles stephensi

Answer

Culex molestus

Question 8

What is true about miliary tuberculosis?

Accepted Answer

Commonly affects liver, kidney and spleen

Answer

Occurs following primary reactivation

Answer

Occurs following secondary reactivation

Answer

Both primary and secondary reactivation

Question 9

A 60-year-old nursing home resident presents with a 3-day history of progressive shortness of breath and cough. The lung examination reveals right basilar crackles. The chest x-ray shows right lower lobe consolidation. Sputum culture grows methicillin-resistant Staphylococcus aureus (MRSA). Select the most appropriate isolation precaution.

Accepted Answer

Contact precautions

Answer

Standard precautions

Answer

Droplet precautions

Answer

Airborne precautions

Question 10

A 6-year-old child presents with perianal pruritus, skin excoriation, and nocturnal enuresis, and is found to be infected with a parasite capable of autoinfection. What is the infective form of this parasite for humans?

Accepted Answer

Embryonated egg

Answer

Filariform larvae

Answer

Adult female

Answer

Adult male

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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