Microbiome — MCQs

Microbiome Indian Medical PG Practice Questions and MCQs

Question 41: Which fungus cannot be grown on artificial media?

A. Rhinosporidium seeberi (Correct Answer)
B. Penicillium marneffei
C. Aspergillus flavus
D. Sporothrix schenckii

Explanation: ### Explanation **Rhinosporidium seeberi** is the correct answer because it is an **obligate parasite** that has never been successfully cultured on artificial (cell-free) media or in continuous cell lines. Despite its name and fungal-like appearance (producing sporangia and spores), molecular phylogenetic analysis has reclassified it as a **Mesomycetozoean**, a group of aquatic protists located at the evolutionary boundary between animals and fungi. #### Analysis of Options: * **Rhinosporidium seeberi (Correct):** It causes Rhinosporidiosis, characterized by friable, leafy, strawberry-like vascular polypoidal masses in the nose or conjunctiva. Diagnosis relies entirely on **histopathology** (demonstrating thick-walled sporangia containing thousands of endospores) because it cannot be grown in vitro. * **Penicillium (Talaromyces) marneffei:** A dimorphic fungus common in Southeast Asia (associated with bamboo rats). It grows readily on Sabouraud Dextrose Agar (SDA), producing a characteristic **diffusible red pigment**. * **Aspergillus flavus:** A common opportunistic mold that grows rapidly on standard laboratory media, producing colonies that are typically lime-green to yellow-green. * **Sporothrix schenckii:** The causative agent of "Rose gardener’s disease." It is a dimorphic fungus that can be cultured on SDA at 25°C (mold form) and on enriched media at 37°C (yeast form). #### High-Yield Clinical Pearls for NEET-PG: * **Habitat:** *R. seeberi* is typically found in stagnant water; infection occurs through traumatized epithelium (e.g., swimming in ponds). * **Staining:** Sporangia stain well with **Gomori Methenamine Silver (GMS)**, PAS, and Mucicarmine. * **Treatment:** Surgical excision with electrocautery of the base is the treatment of choice; Dapsone may be used to prevent recurrence. * **Key Differentiator:** Unlike most fungi, *R. seeberi* does not respond to standard antifungal therapy because of its unique taxonomic position.

Question 42: A 33-year-old female patient presented with brain abscess. On aspiration, the abscess was foul-smelling and yielded red fluorescence under UV light. What is the likely causative organism?

A. Acanthameba
B. Peptostreptococci
C. Bacteroides (Correct Answer)
D. Staphylococcus

Explanation: ### Explanation The correct answer is **Bacteroides** (specifically the *Bacteroides melaninogenicus* group, now reclassified as *Prevotella* and *Porphyromonas*). **1. Why Bacteroides is correct:** The clinical presentation of a **foul-smelling** brain abscess is a classic indicator of an **anaerobic infection**. Among anaerobes, certain species (like *Prevotella* and *Porphyromonas*, formerly categorized under *Bacteroides*) produce **protoporphyrin**, a pigment that causes the colonies or clinical pus to exhibit a characteristic **brick-red fluorescence** when viewed under ultraviolet (UV) light (Wood’s lamp). These organisms are common components of the oral and gingival flora and can reach the brain via hematogenous spread or contiguous extension (e.g., chronic otitis media or sinusitis). **2. Why the other options are incorrect:** * **Acanthamoeba:** Typically causes Granulomatous Amoebic Encephalitis (GAE) in immunocompromised patients. It does not produce a foul smell or red fluorescence. * **Peptostreptococci:** While these are anaerobic cocci often found in brain abscesses and can produce a foul smell, they do not produce the porphyrin pigments required for red fluorescence. * **Staphylococcus:** *S. aureus* is a common cause of brain abscesses (often post-neurosurgery), but it is an aerobe/facultative anaerobe. It produces "creamy" yellow pus, not foul-smelling pus, and does not fluoresce red. **3. NEET-PG High-Yield Pearls:** * **Foul-smelling discharge/pus:** Always think of **Anaerobes**. * **Red Fluorescence:** Pathognomonic for *Prevotella/Porphyromonas* (formerly *Bacteroides melaninogenicus*). * **Black Pigmentation:** On culture (Blood Agar), these organisms develop dark brown to black colonies due to heme sequestration. * **Brain Abscess Source:** Frontal lobe abscesses are often due to **Sinusitis**; Temporal lobe/Cerebellum abscesses are often due to **Otitis Media**.

Question 43: In the gut flora, anaerobic bacteria outnumber the aerobes by what ratio?

A. 10:1
B. 100:1
C. 1000:1 (Correct Answer)
D. 10,000:1

Explanation: **Explanation:** The human gut microbiome is a complex ecosystem dominated by obligate anaerobes. In the large intestine (colon), the environment is strictly anaerobic due to the rapid consumption of oxygen by facultative anaerobes (like *E. coli*) near the mucosal surface. This creates a highly reduced environment that favors the growth of anaerobic species. **1. Why 1000:1 is Correct:** In the colon, the concentration of bacteria is the highest in the body (approx. $10^{11}$ to $10^{12}$ CFU/g of feces). Within this population, **anaerobes outnumber aerobes (facultative anaerobes) by a ratio of 1000:1**. The dominant genera include *Bacteroides*, *Clostridium*, *Bifidobacterium*, and *Fusobacterium*. **2. Analysis of Incorrect Options:** * **10:1 and 100:1:** These ratios significantly underestimate the anaerobic dominance. While these ratios might be seen in the proximal small intestine where oxygen tension is higher and transit time is faster, they do not represent the overall gut flora. * **10,000:1:** While some specific niches in the distal colon might approach very high ratios, the standard established medical consensus for examination purposes is 1000:1. **Clinical Pearls for NEET-PG:** * **Dominant Species:** *Bacteroides fragilis* is the most common anaerobe isolated from intra-abdominal abscesses, though *Bacteroides vulgatus* is more numerous in the normal gut. * **Functions:** The gut flora aids in Vitamin K and B12 synthesis, prevents colonization by pathogens (colonization resistance), and assists in bile acid metabolism. * **Clinical Correlation:** Disruption of this 1000:1 ratio (dysbiosis) by broad-spectrum antibiotics can lead to the overgrowth of *Clostridioides difficile*, resulting in pseudomembranous colitis.

Question 44: HeLa cells are derived from which of the following sources?

A. Sternal bone marrow
B. Embryonal lung
C. Carcinoma of the cervix (Correct Answer)
D. Connective tissue

Explanation: **Explanation:** **HeLa cells** are the first and most widely used **immortalized human cell line** in medical research. They were derived in 1951 from a biopsy of a **cervical adenocarcinoma** (specifically a glandular cancer of the cervix) from a patient named Henrietta Lacks. * **Why Option C is Correct:** HeLa cells are derived from **carcinoma of the cervix**. These cells are unique because they possess an active version of the enzyme **telomerase**, which prevents the gradual shortening of telomeres during cell division. This allows the cells to bypass the Hayflick limit (cellular senescence), making them "immortal" and capable of dividing indefinitely in laboratory cultures. **Analysis of Incorrect Options:** * **Option A (Sternal bone marrow):** This is the source for the **Detroit-6** cell line. * **Option B (Embryonal lung):** Human embryonic lung fibroblasts are used to create **diploid cell lines** like **WI-38** and **MRC-5**, commonly used for vaccine production. * **Option D (Connective tissue):** Cell lines derived from connective tissues (fibroblasts) include the **L-cell line** (mouse subcutaneous tissue). **High-Yield Clinical Pearls for NEET-PG:** * **Viral Association:** HeLa cells contain **HPV-18** DNA, which integrated into the host genome, inactivating the p53 tumor suppressor protein and driving oncogenesis. * **Classification:** HeLa is a **Continuous (Heteroploid) Cell Line**, meaning it has an abnormal number of chromosomes and can be subcultured indefinitely. * **Applications:** They were instrumental in developing the **Salk Polio Vaccine**, studying cancer, gene mapping, and COVID-19 research. * **Other common lines:** **HEp-2** (Laryngeal carcinoma), **Vero** (Monkey kidney), and **BHK-21** (Baby hamster kidney).

Question 45: Which of the toxins produced by Clostridium botulinum is non-neurotoxic?

A. Toxin A
B. Toxin B
C. Toxin C1
D. Toxin C2 (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Toxin C2)** *Clostridium botulinum* produces eight distinct types of toxins, labeled **A through H**. While most are potent neurotoxins that cause flaccid paralysis by inhibiting acetylcholine release, **Toxin C2** is a significant exception. **Why Toxin C2 is the correct answer:** Unlike the neurotropic toxins (A-G), **Toxin C2 is an enterotoxin and a cytotoxin**. It possesses ADP-ribosylating activity that targets cellular actin, leading to increased vascular permeability and tissue damage rather than neurological interference. It does not act on the neuromuscular junction. **Analysis of Incorrect Options:** * **Options A & B (Toxins A and B):** These are the most common causes of human botulism. They are highly potent neurotoxins that proteolytically cleave SNARE proteins (like SNAP-25 or Synaptobrevin), preventing the fusion of acetylcholine vesicles at the synapse. * **Option C (Toxin C1):** While produced by the same strains that produce C2, **C1 is a neurotoxin**. It specifically targets the syntaxin protein at the nerve terminal. --- ### High-Yield NEET-PG Pearls: * **Mechanism of Action:** Botulinum toxins are zinc-dependent endopeptidases that cleave **SNARE proteins**, leading to **flaccid paralysis**. (Contrast with Tetanospasmin, which causes spastic paralysis by inhibiting GABA/Glycine). * **Therapeutic Uses:** Toxin A (Botox) is used for achalasia cardia, strabismus, blepharospasm, and cosmetic wrinkle reduction. * **Infant Botulism:** Associated with **honey** consumption (ingestion of spores), leading to "Floppy Baby Syndrome." * **Food-borne Botulism:** Usually due to ingestion of pre-formed toxins in **canned foods**. * **Detection:** The gold standard for diagnosis is the **Mouse Bioassay** (Mouse Neutralization Test).

Question 46: Giemsa stain of yeast cells in HIV is associated with which of the following infections?

A. Aspergillosis
B. Blastomycosis
C. Cryptococcosis (Correct Answer)
D. Histoplasmosis

Explanation: ### Explanation **Correct Answer: C. Cryptococcosis** The correct answer is **Cryptococcosis** because *Cryptococcus neoformans* is a major opportunistic fungal infection in HIV/AIDS patients (typically when CD4 counts are <100 cells/mm³). While India ink is the classic "negative stain" used to visualize the polysaccharide capsule, **Giemsa stain** is frequently used in cytology (like BAL fluid or CSF) to identify the yeast cells. Under Giemsa, the yeast body stains purple/blue, often surrounded by a characteristic clear "halo" representing the non-staining capsule. **Why other options are incorrect:** * **Aspergillosis:** *Aspergillus* is a mold characterized by **septate hyphae** with acute-angle (45°) branching, not yeast cells. It is typically visualized using Silver stains (GMS) or PAS. * **Blastomycosis:** While it presents as yeast, *Blastomyces* is characterized by **broad-based budding**. It is less commonly associated with HIV compared to Cryptococcus and is usually identified via KOH mount or H&E stain. * **Histoplasmosis:** This is a significant distractor. *Histoplasma capsulatum* also appears as small intracellular yeasts on Giemsa stain (found within macrophages). However, in the context of standard NEET-PG questions, if the focus is on the **classic yeast cell identification in HIV-associated meningitis/systemic infection**, *Cryptococcus* is the primary association. **NEET-PG High-Yield Pearls:** * **Cryptococcus:** Most common cause of fungal meningitis in HIV. Look for "Soap bubble appearance" on MRI brain. * **Capsule:** The only encapsulated fungus. The capsule is composed of **Glucuronoxylomannan**. * **Staining Summary:** * **India Ink:** Best for capsule visualization (Negative staining). * **Mucicarmine:** Stains the capsule bright red (Specific). * **Fontana-Masson:** Stains melanin in the cell wall black. * **Antigen Detection:** CrAg (Cryptococcal Antigen) via Lateral Flow Assay (LFA) is now the gold standard for rapid diagnosis.

Question 47: Japanese B encephalitis virus is transmitted by which vector?

A. Culex tritaeniorhynchus (Correct Answer)
B. Hard tick
C. Culex fatigans
D. Soft tick

Explanation: ### Explanation **Correct Option: A. Culex tritaeniorhynchus** Japanese Encephalitis (JE) is caused by a Group B Arbovirus (Flavivirus). In India and Southeast Asia, the primary vector is **Culex tritaeniorhynchus**. These mosquitoes are "rice-field breeders" and are zoophilic, meaning they prefer feeding on animals (pigs and water birds) rather than humans. Humans are **accidental, dead-end hosts** because the viremia in humans is insufficient to infect a biting mosquito. **Analysis of Incorrect Options:** * **B & D (Hard and Soft Ticks):** Ticks are vectors for diseases like Kyasanur Forest Disease (KFD), Crimean-Congo Hemorrhagic Fever (CCHF), and Rickettsial infections. They do not transmit the JE virus. * **C (Culex fatigans):** Also known as *Culex quinquefasciatus*, this is the primary vector for **Bancroftian Filariasis**. While it belongs to the same genus, it is not the principal vector for Japanese Encephalitis. **High-Yield Clinical Pearls for NEET-PG:** * **Reservoir/Amplifier Host:** The **Pig** is the most important amplifier host (shows no symptoms but high viremia). * **Natural Host:** Ardeid birds (Paddy birds, Herons). * **Seasonality:** JE outbreaks usually coincide with the rainy season and intensive rice cultivation. * **Vaccination:** The most commonly used vaccine in the National Immunization Schedule is the **SA-14-14-2** (Live attenuated, Chinese origin), given at 9 months and 16–24 months. * **Diagnosis:** **IgM Capture ELISA** (MAC-ELISA) of CSF or serum is the gold standard.

Question 48: Which one of the following infections is typically seen one month after renal transplantation?

A. Varicella-zoster virus (Chickenpox)
B. Coxsackie virus
C. Cytomegalovirus (Correct Answer)
D. Epstein-Barr virus

Explanation: **Explanation:** Infections in solid organ transplant (SOT) recipients follow a predictable chronological pattern based on the level of immunosuppression. This timeline is divided into three phases: 1. **Early Phase (<1 month):** Infections are usually related to surgical complications or donor-derived infections (e.g., MRSA, HSV). 2. **Middle Phase (1–6 months):** This is the period of maximal immunosuppression. **Cytomegalovirus (CMV)** is the most common and significant opportunistic infection during this window, typically peaking between **1 to 4 months** post-transplant. It manifests as fever, leukopenia, or tissue-invasive disease (pneumonitis, hepatitis, or colitis). 3. **Late Phase (>6 months):** Community-acquired infections or chronic viral infections (e.g., VZV, BK virus) predominate. **Analysis of Options:** * **Cytomegalovirus (Correct):** The classic "middle period" pathogen. It is often a reactivation of latent virus triggered by immunosuppressive therapy (especially ATG or high-dose steroids). * **Varicella-zoster virus (VZV):** While it can occur, it typically appears much later in the post-transplant course (usually >6 months) as a result of waning T-cell immunity. * **Epstein-Barr virus (EBV):** While it can occur in the middle phase, it is more specifically associated with **Post-Transplant Lymphoproliferative Disorder (PTLD)** rather than a routine acute infection at the one-month mark. * **Coxsackie virus:** This is a community-acquired enterovirus not specifically linked to the post-transplant immunosuppression timeline. **High-Yield Clinical Pearls for NEET-PG:** * **CMV Diagnosis:** The gold standard for monitoring is **Quantitative PCR** for CMV DNA. * **Histology:** Look for **"Owl’s eye"** intranuclear inclusion bodies. * **Prophylaxis:** **Valganciclovir** is commonly used to prevent CMV infection in high-risk transplant patients. * **BK Virus:** Another high-yield renal transplant virus; it causes **nephropathy** and ureteral stenosis, usually 3–6 months post-transplant.

Question 49: Vincent's angina is caused by Borrelia vincenti along with which of the following microorganisms?

A. Lactobacillus
B. Peptostreptococcus
C. Fusobacterium (Correct Answer)
D. Bacteroides

Explanation: **Explanation:** **Vincent’s Angina** (also known as Trench Mouth or Acute Necrotizing Ulcerative Gingivitis - ANUG) is a painful, non-contagious infection of the gums and throat characterized by ulceration, pseudomembrane formation, and "halitosis" (foul breath). The underlying medical concept is **synergistic polymicrobial infection**. It is caused by a symbiotic relationship between two specific organisms: 1. **_Borrelia vincentii_** (a large Gram-negative spirochete). 2. **_Fusobacterium fusiforme_** (a Gram-negative anaerobic spindle-shaped rod). This combination is often referred to as the **"fusospirochetal complex."** These organisms are part of the normal oral flora but become pathogenic under conditions of poor oral hygiene, malnutrition, or extreme stress. **Analysis of Options:** * **Fusobacterium (Correct):** It acts synergistically with the spirochete to cause tissue necrosis and ulceration. * **Lactobacillus:** These are Gram-positive rods that are part of the normal vaginal and GI flora; they are primarily associated with dental caries, not necrotizing gingivitis. * **Peptostreptococcus:** While these are anaerobic cocci found in the mouth, they are more commonly associated with aspiration pneumonia or deep tissue abscesses rather than the specific clinical entity of Vincent’s angina. * **Bacteroides:** Although anaerobic, they are typically associated with intra-abdominal infections (e.g., *B. fragilis*) rather than the classic fusospirochetal lesions of the oropharynx. **Clinical Pearls for NEET-PG:** * **Diagnosis:** Primarily clinical, but a Gram stain of the ulcer shows the characteristic "spirochetes and fusiform bacilli." * **Clinical Presentation:** Punched-out interdental papillae covered by a gray pseudomembrane. * **Treatment:** Penicillin is the drug of choice, along with metronidazole and surgical debridement. * **Historical Note:** It was called "Trench Mouth" due to its high prevalence among soldiers in the trenches during World War I.

Question 50: What is the causative agent of redleg disease?

A. Pseudomonas
B. Moudly sugar cane fibre
C. Coniosporium
D. Aeromonas (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Aeromonas** Redleg disease (also known as bacterial septicemia) is a common and often fatal infection primarily affecting amphibians, such as frogs and toads. The causative agent is **Aeromonas hydrophila**, a Gram-negative, facultative anaerobic rod commonly found in freshwater environments. The disease is characterized by cutaneous hemorrhages, particularly on the ventral surface of the legs and abdomen (giving it the name "redleg"), resulting from systemic vasculitis and sepsis. In humans, *Aeromonas* is clinically significant as a cause of gastroenteritis and wound infections, especially following exposure to contaminated water. **Analysis of Incorrect Options:** * **A. Pseudomonas:** While *Pseudomonas* species can cause various opportunistic infections in aquatic animals and humans (like "swimmer’s ear" or ecthyma gangrenosum), they are not the specific primary cause of the classic "redleg" syndrome. * **B. Mouldy sugar cane fibre:** This is the causative agent of **Bagassosis**, an occupational hypersensitivity pneumonitis caused by the inhalation of *Thermoactinomyces sacchari* found in stored, mouldy sugar cane. * **C. Coniosporium:** Specifically *Coniosporium corticale*, this fungus is associated with **Maple bark stripper’s disease**, another form of hypersensitivity pneumonitis. **High-Yield Clinical Pearls for NEET-PG:** * **Aeromonas hydrophila** is oxidase-positive, which distinguishes it from the Enterobacteriaceae family. * It is a well-known cause of **medicinal leech-associated infections**; prophylactic antibiotics are often required during leech therapy. * **Zoonotic link:** While redleg is an animal disease, *Aeromonas* can cause severe necrotizing fasciitis in immunocompromised humans following water-related trauma.

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