Microbiome Practice Questions

Q: Which one of the following is diagnosed by latex agglutination for antigen in CSF?

Cryptococcus. **Explanation:** **1. Why Cryptococcus is Correct:** The diagnosis of **Cryptococcal meningitis** relies heavily on the detection of the capsular polysaccharide antigen (GXM) in the Cerebrospinal Fluid (CSF) or serum. The **Latex Agglutination (LA) test** uses latex beads coated with specific antibodies that clump (agglutinate) in the presence of this antigen. It is highly sensitive (>90%) and specific, often providing a diagnosis faster than a fungal culture and being more sensitive than an India Ink preparation (which can be negative in early or low-burden infections). **2. Why the Other Options are Incorrect:** * **Aspergillosis:** Diagnosis typically involves the **Galactomannan** antigen assay (via ELISA) or Beta-D-Glucan testing, primarily in serum or Bronchoalveolar Lavage (BAL) fluid, rather than CSF latex agglutination. * **Candida:** While Beta-D-Glucan is a marker for invasive candidiasis, there is no routine, reliable latex agglutination test for Candida antigen in the CSF used for clinical diagnosis. * **Histoplasmosis:** Diagnosis is usually made via **Urinary or Serum antigen** detection (ELISA) or histopathology showing intracellular yeast in macrophages. CSF involvement is rare and not typically diagnosed via LA. **3. High-Yield Clinical Pearls for NEET-PG:** * **India Ink:** Shows a "clear halo" (capsule) against a dark background but is less sensitive than the LA test. * **Culture:** Sabouraud Dextrose Agar (SDA) is used; colonies appear creamy/mucoid. * **Stains:** **Mucicarmine** is specific for the capsule (stains it red); **Masson-Fontana** stains the melanin in the cell wall. * **Risk Factor:** Strongly associated with low CD4 counts (<100 cells/µL) in HIV/AIDS patients.

Q: What is the definitive host for Taenia solium?

Man. **Explanation:** In parasitology, the **definitive host** is defined as the host in which the parasite undergoes its sexual reproductive cycle or reaches the adult stage. For *Taenia solium* (the pork tapeworm), **Man** is the only definitive host. Humans harbor the adult tapeworm in the small intestine after ingesting undercooked pork containing **cysticerci** (larval stage). **Analysis of Options:** * **Option B (Man):** Correct. Humans serve as the definitive host when they harbor the adult worm. Notably, humans can also act as an **accidental intermediate host** if they ingest *T. solium* eggs (via feco-oral route), leading to **Cysticercosis**. * **Option A (Pig):** Incorrect. The pig is the **intermediate host**. It ingests eggs from human feces, and the larvae (Cysticercus cellulosae) develop in the pig's muscle tissue. * **Option C (Cattle):** Incorrect. Cattle serve as the intermediate host for ***Taenia saginata*** (beef tapeworm). *T. saginata* does not cause cysticercosis in humans. * **Option D (Dog):** Incorrect. Dogs are the definitive hosts for ***Echinococcus granulosus*** (Hydatid disease), while humans are the accidental intermediate hosts. **Clinical Pearls for NEET-PG:** * **Infective stage for Intestinal Taeniasis:** Cysticercus cellulosae (larva in pork). * **Infective stage for Cysticercosis:** Eggs of *T. solium* (passed in human feces). * **Neurocysticercosis (NCC):** The most common cause of adult-onset seizures worldwide; characterized by "starry sky" appearance on MRI/CT. * **Diagnostic Tip:** *T. solium* has a scolex with four suckers and a rostellum with hooks ("armed" tapeworm), whereas *T. saginata* lacks hooks ("unarmed").

Q: A 2-year-old child presents with fever and vomiting, exhibiting neck rigidity. Cerebrospinal fluid analysis reveals over 2000 polymorphs/mm³, protein of 100 mg/dL, and glucose of 10 mg/dL. Gram stain shows Gram-negative coccobacilli, and culture demonstrates growth only on chocolate agar, not on blood agar. What is the causative agent?

Haemophilus influenzae. ### Explanation The clinical presentation of fever, neck rigidity, and CSF findings (high polymorphs, high protein, and very low glucose) is diagnostic of **Acute Bacterial Meningitis**. **Why Haemophilus influenzae is correct:** The key diagnostic clue is the growth pattern: **growth on chocolate agar but not on blood agar**. *H. influenzae* is a fastidious Gram-negative coccobacillus that requires two specific growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. * **Blood Agar:** Contains Factor X but lacks Factor V (as NAD is sequestered inside RBCs and inactivated by blood enzymes). Hence, *H. influenzae* cannot grow alone on blood agar (unless "satellitism" occurs with *S. aureus*). * **Chocolate Agar:** Prepared by heating blood, which lyses RBCs to release both Factor X and V and inactivates the NAD-destroying enzymes, allowing *H. influenzae* to thrive. **Analysis of Incorrect Options:** * **Neisseria meningitidis (A):** While a common cause of meningitis, it is a Gram-negative **diplococcus** (kidney-bean shaped) and can grow on both blood agar and chocolate agar. * **Branhamella (Moraxella) catarrhalis (C):** This is a Gram-negative diplococcus primarily associated with otitis media and respiratory infections, not typically meningitis. It grows well on routine blood agar. * **Legionella pneumophila (D):** This causes pneumonia (Legionnaires' disease), not meningitis. It requires **BCYE (Buffered Charcoal Yeast Extract) agar** for growth, not chocolate agar. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows on blood agar only near colonies of *Staphylococcus aureus*, which provide the necessary Factor V. * **Quellung Reaction:** Positive for *H. influenzae* type b (Hib) due to its polysaccharide capsule. * **Vaccination:** The Hib conjugate vaccine has significantly reduced the incidence of meningitis in children under 5.

Q: Which one of the following is NOT a dimorphic fungus?

Cryptococcus. **Explanation:** The core concept tested here is the classification of fungi based on their morphology. **Dimorphic fungi** are unique because they exist in two distinct forms depending on environmental conditions (primarily temperature): they grow as **molds** (hyphae) in the environment/cold (25°C) and as **yeasts** in the human body/heat (37°C). **Why Cryptococcus is the correct answer:** * **Cryptococcus neoformans** is a **monomorphic yeast**. It exists strictly as an encapsulated yeast both in the environment and in human tissue. It does not produce a mold form at lower temperatures, which distinguishes it from the true systemic dimorphic pathogens. **Analysis of Incorrect Options (Dimorphic Fungi):** * **Histoplasma capsulatum:** A classic dimorphic fungus (the "Ohio Valley Fever" agent). It appears as small intracellular yeasts within macrophages at 37°C. * **Coccidioides immitis:** A dimorphic fungus that exists as mold in soil but forms unique **spherules** filled with endospores (rather than simple yeasts) in the lungs at 37°C. * **Blastomyces dermatitidis:** A dimorphic fungus characterized by "Broad-Based Budding" yeasts at 37°C. **High-Yield NEET-PG Pearls:** 1. **Mnemonic for Dimorphic Fungi:** *"Body Heat Probably Changes Shape"* (**B**lastomyces, **H**istoplasma, **P**aracoccidioides, **C**occidioides, **S**porothrix). Note: *Talaromyces (Penicillium) marneffei* is also dimorphic. 2. **Cryptococcus Key Feature:** It is the only medically important fungus with a **polysaccharide capsule**, best visualized using **India Ink** (negative staining) or Latex Agglutination. 3. **Temperature Rule:** "Mold in the Cold, Yeast in the Beast."

Q: Tick-borne viral encephalitis is seen in which of the following conditions?

Omsk hemorrhagic fever. **Explanation:** **Omsk Hemorrhagic Fever (OHF)** is the correct answer because it is a viral disease caused by the OHF virus (Family: *Flaviviridae*), which is transmitted primarily by the bite of infected **Dermacentor ticks** (specifically *D. reticulatus*). While the disease is characterized by fever and hemorrhage, it is clinically unique among hemorrhagic fevers for its frequent involvement of the Central Nervous System (CNS), leading to **meningoencephalitis** in a significant number of cases. **Analysis of Incorrect Options:** * **Lassa Fever:** Caused by the Lassa virus (Family: *Arenaviridae*), it is transmitted via contact with the excreta of the **Mastomys rat** (multimammate rat). It is not tick-borne. * **Marburg & Ebola Viruses:** Both belong to the **Filoviridae** family. They are transmitted through direct contact with infected blood/body fluids or via fruit bats (*Pteropodidae*). They cause severe viral hemorrhagic fevers but are not transmitted by ticks. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Dermacentor reticulatus* tick is the primary vector for OHF; however, muskrats can act as incidental hosts. * **Biphasic Course:** OHF often presents with a biphasic illness—the first phase is flu-like, followed by a second phase that may involve encephalitis. * **Other Tick-Borne Encephalitis (TBE) Viruses:** Include Kyasanur Forest Disease (KFD) in India (transmitted by *Haemaphysalis* ticks) and Powassan virus. * **Key Distinction:** Always associate *Flaviviruses* (like OHF and KFD) with tick/mosquito vectors, whereas *Filoviruses* (Ebola/Marburg) and *Arenaviruses* (Lassa) are associated with bats and rodents, respectively.

Q: Lyme disease is caused by which bacterium?

Borrelia burgdorferi. **Explanation:** **Lyme disease** is a multisystem inflammatory disorder caused by the spirochete **Borrelia burgdorferi**. It is primarily transmitted to humans through the bite of infected **Ixodes ticks** (deer ticks). The disease typically progresses through three stages: early localized (characterized by the pathognomonic **Erythema Chronicum Migrans** or "bull’s eye" rash), early disseminated (neurological and cardiac involvement), and late disseminated (chronic arthritis). **Analysis of Options:** * **Borrelia burgdorferi (Correct):** The primary causative agent of Lyme disease in North America and parts of Europe. * **Borrelia recurrentis:** This species causes **Epidemic Relapsing Fever**, which is transmitted by the human body louse (*Pediculus humanus corporis*). * **Borrelia vincentii:** Along with *Fusobacterium*, this organism is associated with **Vincent’s Angina** (trench mouth) and Cancrum Oris (Noma). * **Leptospira:** This genus causes **Leptospirosis** (Weil’s disease), typically transmitted through contact with water or soil contaminated by the urine of infected animals (rodents). **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes scapularis* (nymph stage is the most common transmitter). * **Reservoir:** White-footed mouse (*Peromyscus leucopus*). * **Diagnosis:** Two-tier testing is recommended—**ELISA** followed by a confirmatory **Western Blot**. * **Treatment:** **Doxycycline** is the drug of choice for early stages. For pregnant women or children <8 years, **Amoxicillin** is used. Ceftriaxone is preferred for neurological or cardiac manifestations. * **Microscopy:** Borrelia are the only spirochetes large enough to be visualized under a light microscope using **Giemsa or Wright stain**.

Q: Which of the following is NOT a characteristic finding in viral encephalitis?

Astroglial proliferation. **Explanation:** In viral encephalitis, the hallmark pathological changes are primarily related to **acute inflammation** and **direct viral cytopathic effects**. **1. Why "Astroglial proliferation" is the correct answer:** Astroglial proliferation (astrogliosis) is a feature of **chronic** brain injury, repair, or "scarring" (gliosis) in the CNS. While it may occur as a late-stage sequela after the acute infection has subsided, it is **not** a characteristic finding of the acute phase of viral encephalitis. Acute viral infections typically present with microglial nodules (the CNS's primary immune responders) rather than immediate macroglial (astrocyte) proliferation. **2. Analysis of Incorrect Options:** * **Perivascular mononuclear infiltrate (Option B):** This is a classic histological hallmark of viral encephalitis, often referred to as **"perivascular cuffing."** Lymphocytes, plasma cells, and monocytes aggregate around blood vessels in the Virchow-Robin spaces. * **Inclusion bodies (Option C):** Many viruses produce characteristic inclusions. Examples include **Negri bodies** (intracytoplasmic) in Rabies and **Cowdry Type A** (intranuclear) in Herpes Simplex Virus (HSV). These are vital diagnostic clues in pathology. **High-Yield Clinical Pearls for NEET-PG:** * **HSV-1 Encephalitis:** Most common cause of sporadic fatal encephalitis; typically involves the **temporal lobes**. * **Microglial Nodules:** Small clusters of microglia around areas of necrosis; a pathognomonic feature of viral CNS infection. * **Neuronophagia:** The process where phagocytes (microglia) ingest necrotic neurons during acute viral infection. * **Japanese Encephalitis:** The most common cause of epidemic viral encephalitis in India; often involves the **thalamus and basal ganglia**.

Q: Which of the following is NOT a common cause of meningitis in adults?

Streptococcus pyogenes (Group A). **Explanation:** The correct answer is **Streptococcus pyogenes (Group A)**. While *S. pyogenes* is a major cause of skin and soft tissue infections (cellulitis, necrotizing fasciitis) and pharyngitis, it is an **extremely rare** cause of meningitis in adults. When it does occur, it is usually secondary to a direct extension from a nearby focus, such as otitis media or mastoiditis, rather than primary hematogenous spread. **Analysis of Incorrect Options:** * **Group B Streptococcus (GBS):** While primarily known as the leading cause of neonatal meningitis, GBS is an increasingly recognized cause of meningitis in **older adults** and those with comorbidities like diabetes or malignancy. * **Mycobacterium tuberculosis:** Tuberculous meningitis (TBM) is a significant cause of chronic meningitis in adults, especially in endemic regions like India. It typically presents with a subacute course and basal exudates. * **Staphylococcus aureus / Haemophilus influenzae:** *S. aureus* is a common cause of post-neurosurgical or trauma-related meningitis. *H. influenzae* (Type B), though reduced by vaccination, remains a cause of adult meningitis, particularly in non-immunized or immunocompromised individuals. **NEET-PG High-Yield Pearls:** * **Most common cause of adult bacterial meningitis:** *Streptococcus pneumoniae* (all ages except neonates). * **Most common cause in neonates:** *Group B Streptococcus* (followed by *E. coli* and *Listeria*). * **Waterhouse-Friderichsen Syndrome:** Adrenal hemorrhage associated with *Neisseria meningitidis*. * **CSF Findings in Bacterial Meningitis:** High protein, low glucose (<40 mg/dL), and marked neutrophilic pleocytosis.

Question 1

Which one of the following is diagnosed by latex agglutination for antigen in CSF?

Accepted Answer

Cryptococcus

Answer

Aspergillosis

Answer

Candida

Answer

Histoplasmosis

Question 2

A 3-week-old child presented with meningitis. A presumptive diagnosis of late onset of a perinatal infection was made. The CSF culture was positive for gram-positive bacilli. Which of the following characteristics of this bacterium would be helpful in differentiating it from other bacterial agents?

Accepted Answer

Motility at 25°C

Answer

Ability to grow on blood agar

Answer

Ability to produce catalase

Answer

Fermentative attack on sugars

Question 3

What is the definitive host for Taenia solium?

Accepted Answer

Man

Answer

Pig

Answer

Cattle

Answer

Dog

Question 4

A patient with a history of previous Hepatitis B infection and now immune to it will show which of the following serological markers?

Accepted Answer

Hepatitis B surface antigen (HBsAg) negative, Antibody to HBsAg (Anti-HBsAg) positive, Antibody to Hepatitis B core antigen (Anti-HBcAg) negative

Answer

Hepatitis B surface antigen (HBsAg) positive, Antibody to HBsAg (Anti-HBsAg) positive, Antibody to Hepatitis B core antigen (Anti-HBcAg) positive

Answer

Hepatitis B surface antigen (HBsAg) negative, Antibody to HBsAg (Anti-HBsAg) negative, Antibody to Hepatitis B core antigen (Anti-HBcAg) negative

Answer

Hepatitis B surface antigen (HBsAg) negative, Antibody to HBsAg (Anti-HBsAg) negative, Antibody to Hepatitis B core antigen (Anti-HBcAg) positive

Question 5

A 2-year-old child presents with fever and vomiting, exhibiting neck rigidity. Cerebrospinal fluid analysis reveals over 2000 polymorphs/mm³, protein of 100 mg/dL, and glucose of 10 mg/dL. Gram stain shows Gram-negative coccobacilli, and culture demonstrates growth only on chocolate agar, not on blood agar. What is the causative agent?

Accepted Answer

Haemophilus influenzae

Answer

Neisseria meningitidis

Answer

Branhamella catarrhalis

Answer

Legionella pneumophila

Question 6

Which one of the following is NOT a dimorphic fungus?

Accepted Answer

Cryptococcus

Answer

Histoplasma

Answer

Coccidioidomycosis

Answer

Blastomycosis

Question 7

Tick-borne viral encephalitis is seen in which of the following conditions?

Accepted Answer

Omsk hemorrhagic fever

Answer

Lassa fever

Answer

Marburg fever

Answer

Ebola virus

Question 8

Lyme disease is caused by which bacterium?

Accepted Answer

Borrelia burgdorferi

Answer

Borrelia recurrentis

Answer

Borrelia vincentis

Answer

Leptospira

Question 9

Which of the following is NOT a characteristic finding in viral encephalitis?

Accepted Answer

Astroglial proliferation

Answer

Perivascular mononuclear infiltrate

Answer

Inclusion bodies, intranuclear and intracytoplasmic

Answer

None of the above

Question 10

Which of the following is NOT a common cause of meningitis in adults?

Accepted Answer

Streptococcus pyogenes (Group A)

Answer

Group B Streptococcus

Answer

Mycobacterium tuberculosis

Answer

Staphylococcus aureus/Haemophilus influenzae

Microbiome — MCQs

Microbiome — MCQs

On this page

Practice by Chapter

Want unlimited practice?