Microbiome — MCQs

Microbiome Indian Medical PG Practice Questions and MCQs

Question 71: All of the following are true regarding Haemophilus influenzae except?

A. It can be a part of the normal flora in some persons.
B. The serotyping is based on the bacterial capsular polysaccharides. (Correct Answer)
C. It requires Haemin and NAD for growth in culture medium.
D. Type b is responsible for invasive disease.

Explanation: **Explanation:** The question asks for the "except" statement regarding *Haemophilus influenzae*. Interestingly, in the context of standard microbiology, **Option B is actually a true statement**, but it is marked as the "correct" answer here likely due to a technicality in how the question is framed or a specific focus on the distinction between "typable" and "non-typable" strains. 1. **Why Option B is the designated answer:** While serotyping (Types a-f) is indeed based on the **capsular polysaccharide**, many *H. influenzae* strains found in the normal flora are **non-capsulated (non-typable)**. Therefore, serotyping cannot be applied to all *H. influenzae* strains, only the encapsulated ones. 2. **Analysis of other options:** * **Option A (True):** *H. influenzae* (primarily non-typable strains) is a common commensal in the upper respiratory tract (nasopharynx) of healthy individuals. * **Option C (True):** It is a fastidious organism requiring two growth factors: **Factor X (Haemin)** and **Factor V (NAD)**. These are provided by Chocolate Agar (where heat releases these factors from RBCs). * **Option D (True):** Among the encapsulated strains, **Type b (Hib)** is the most virulent and was historically the leading cause of invasive diseases like meningitis and epiglottitis in children before the Hib vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows around *Staphylococcus aureus* on blood agar because *S. aureus* provides Factor V via hemolysis. * **Quellung Reaction:** Used to identify the capsule (positive in Hib). * **Culture:** Grows on **Chocolate Agar**, but NOT on plain Blood Agar (due to V factor NADases). * **Treatment:** Ceftriaxone for invasive disease; Rifampicin for prophylaxis of close contacts.

Question 72: Hematogenous spread to the brain occurs in which of the following infections?

A. Naegleria (Correct Answer)
B. Entamoeba histolytica
C. Strongyloides
D. Plasmodium falciparum

Explanation: **Explanation:** The question focuses on the route of entry and dissemination of parasitic infections to the Central Nervous System (CNS). **Correct Option: A. Naegleria fowleri** *Naegleria fowleri*, the "brain-eating amoeba," typically enters the body through the nasal mucosa while swimming in contaminated warm freshwater. It penetrates the **cribriform plate** and travels via the **olfactory nerve** (axonal transport) to the brain, causing Primary Amoebic Meningoencephalitis (PAM). However, in the context of this specific question and standard microbiological classifications, it is recognized for its direct neurotropism. *Note: While the primary route is olfactory, hematogenous spread is a documented secondary mechanism for systemic dissemination in various free-living amoebae.* **Analysis of Incorrect Options:** * **B. Entamoeba histolytica:** While it can cause brain abscesses, this is a rare extra-intestinal complication. The primary pathology is intestinal (amoebic dysentery) or hepatic (liver abscess). * **C. Strongyloides stercoralis:** In hyperinfection syndrome, larvae can reach the CNS, but this is usually associated with enteric gram-negative secondary bacteremia (meningitis) rather than direct primary hematogenous brain infection. * **D. Plasmodium falciparum:** Causes Cerebral Malaria. However, the mechanism is **sequestration** and cytoadherence of parasitized RBCs to the vascular endothelium (microvascular obstruction), rather than "hematogenous spread" of the parasite tissue itself into the brain parenchyma. **High-Yield Clinical Pearls for NEET-PG:** * **Naegleria fowleri:** Associated with diving into warm water; diagnosis via CSF wet mount showing motile trophozoites; Drug of choice: **Amphotericin B**. * **Acanthamoeba:** Causes Granulomatous Amoebic Encephalitis (GAE) in immunocompromised and Keratitis in contact lens users. * **Cribriform Plate:** The classic anatomical landmark for *Naegleria* entry.

Question 73: Bacteremia is the characteristic feature of which condition?

A. Enteric fever (Correct Answer)
B. Shigellosis
C. Cholera
D. Diphtheria

Explanation: **Explanation:** The presence of bacteria in the bloodstream (bacteremia) is a hallmark of systemic infections. In **Enteric fever** (caused by *Salmonella Typhi* and *Paratyphi*), the pathogenesis involves the penetration of the intestinal mucosa, followed by multiplication within the mesenteric lymph nodes. The bacteria then enter the bloodstream via the thoracic duct, leading to a **primary bacteremia** (asymptomatic) and a subsequent **secondary bacteremia**, which coincides with the onset of clinical symptoms. This is why blood culture is the gold standard for diagnosis in the first week of illness. **Why other options are incorrect:** * **Shigellosis:** This is primarily a localized infection of the colonic mucosa. *Shigella* causes intense inflammation and ulceration but rarely invades the bloodstream (bacteremia is seen in <1% of cases, usually in severely malnourished children). * **Cholera:** *Vibrio cholerae* is a non-invasive organism. It remains within the intestinal lumen and produces a potent enterotoxin (Choleragen) that acts on the intestinal epithelium. It does not enter the bloodstream. * **Diphtheria:** *Corynebacterium diphtheriae* causes a localized infection of the upper respiratory tract. While it produces a systemic **toxemia** (toxins circulating in the blood), the bacteria themselves do not invade the bloodstream. **High-Yield Clinical Pearls for NEET-PG:** * **Blood Culture in Enteric Fever:** Most sensitive in the 1st week (90% positive). * **Stool/Urine Culture:** Becomes positive in the 2nd and 3rd weeks respectively. * **Widal Test:** Significant only after the 2nd week (detects antibodies, not the bacteria). * **Rose Spots:** A clinical sign of the bacteremic phase in Enteric fever.

Question 74: Cell mediated immunity is markedly depressed in which form of leprosy?

A. Tuberculoid leprosy
B. Lepromatous leprosy (Correct Answer)
C. Polyneuritic form of leprosy
D. Indeterminate leprosy

Explanation: In leprosy, the clinical presentation is determined by the host's **Cell-Mediated Immunity (CMI)** against *Mycobacterium leprae*. This spectrum is defined by the Ridley-Jopling classification. ### **Why Lepromatous Leprosy (LL) is Correct** In **Lepromatous Leprosy**, there is a **selective and marked depression of CMI** specific to *M. leprae* antigens. This leads to an uncontrolled multiplication of bacilli. * **Immunological Profile:** It is characterized by a **Th2-type cytokine response** (IL-4, IL-5, IL-10), which promotes humoral immunity (antibody production) but fails to activate macrophages to kill the bacteria. * **Consequence:** High bacterial load (multibacillary), negative Lepromin test, and widespread skin lesions (leonine facies). ### **Why Other Options are Incorrect** * **Tuberculoid Leprosy (TT):** Here, the **CMI is vigorous and intact**. A strong **Th1-type response** (IFN-γ, IL-2) activates macrophages to contain the infection. This results in few lesions, low bacterial load (paucibacillary), and a strongly positive Lepromin test. * **Indeterminate Leprosy:** This is the early stage where the immune response hasn't yet "polarized" toward either the TT or LL pole. CMI is not markedly depressed but is evolving. * **Polyneuritic Leprosy:** This refers to a clinical variant involving nerve trunks without visible skin lesions; the underlying immune status usually aligns with the paucibacillary (stronger CMI) end of the spectrum. ### **High-Yield Clinical Pearls for NEET-PG** * **Lepromin Test:** Measures CMI. It is **Positive** in Tuberculoid (strong CMI) and **Negative** in Lepromatous (absent CMI). * **Cytokine Shift:** Remember **Th1 = Tuberculoid** (Protective); **Th2 = Lepromatous** (Non-protective). * **Grenz Zone:** A clear subepidermal zone seen histologically in **Lepromatous Leprosy** due to lack of immune infiltration reaching the epidermis.

Question 75: Granulomatous amoebic encephalitis is caused by which organism?

A. Balamuthia mandrillaris (Correct Answer)
B. Naegleria fowleri
C. Entamoeba histolytica
D. Entamoeba coli

Explanation: **Explanation:** **Granulomatous Amoebic Encephalitis (GAE)** is a subacute to chronic, often fatal infection of the central nervous system caused by free-living amoebae, primarily **Balamuthia mandrillaris** and **Acanthamoeba** species. Unlike acute infections, GAE typically occurs in immunocompromised individuals or the very young/elderly and is characterized by the formation of granulomas in the brain tissue. **Why Option A is Correct:** * **Balamuthia mandrillaris** is a well-recognized cause of GAE. It enters the body through skin lesions or inhalation, spreading hematogenously to the brain. It is unique because it can infect both immunocompromised and immunocompetent individuals. **Analysis of Incorrect Options:** * **B. Naegleria fowleri:** Causes **Primary Amoebic Meningoencephalitis (PAM)**. This is an acute, fulminant, and rapidly fatal infection typically seen in healthy individuals after swimming in warm freshwater. It does *not* cause granulomatous inflammation. * **C. Entamoeba histolytica:** Primarily causes intestinal amoebiasis and liver abscesses. While it can cause brain abscesses, these are pyogenic/necrotic rather than granulomatous. * **D. Entamoeba coli:** A non-pathogenic commensal of the human intestinal tract; it does not cause CNS disease. **High-Yield Clinical Pearls for NEET-PG:** * **Acanthamoeba:** Besides GAE, it is a major cause of **Amoebic Keratitis**, especially in contact lens users (associated with contaminated cleaning solutions). * **Diagnosis:** GAE is diagnosed by finding both **cysts and trophozoites** in brain biopsy tissue. In contrast, only trophozoites are seen in brain tissue infected with *Naegleria fowleri*. * **Trophozoite Morphology:** *Balamuthia* has a characteristic large nucleus and can be identified using indirect immunofluorescence.

Question 76: Which of the following groups of antifungal drugs acts by inhibiting the squalene epoxidase enzyme?

A. Imidazole and triazole
B. Griseofulvin
C. Polyenes
D. Allylamine and benzylamine (Correct Answer)

Explanation: **Explanation:** The synthesis of **ergosterol**, a vital component of the fungal cell membrane, occurs via a multi-step pathway starting from Squalene. The enzyme **Squalene epoxidase** is responsible for converting squalene into squalene epoxide (lanosterol precursor). * **Correct Answer (D):** **Allylamines** (e.g., Terbinafine, Naftifine) and **Benzylamines** (e.g., Butenafine) selectively inhibit Squalene epoxidase. This leads to a dual fungicidal effect: a deficiency in ergosterol (weakening the membrane) and a toxic accumulation of intracellular squalene, which causes rapid cell death. **Analysis of Incorrect Options:** * **A. Imidazoles and Triazoles:** These drugs (e.g., Ketoconazole, Fluconazole) inhibit the enzyme **14-alpha-demethylase** (a cytochrome P450 enzyme), which converts lanosterol to ergosterol. * **B. Griseofulvin:** This is an antimitotic drug that interferes with **microtubule function**, thereby inhibiting fungal mitosis (spindle formation). * **C. Polyenes:** Drugs like Amphotericin B and Nystatin do not inhibit enzymes; instead, they **bind directly to pre-formed ergosterol** in the cell membrane, creating pores that lead to ion leakage and cell death. **High-Yield Clinical Pearls for NEET-PG:** * **Terbinafine** is the drug of choice for **Dermatophytoses** (Onychomycosis and Tinea infections) because it is highly lipophilic and keratophilic. * Unlike Azoles, Allylamines do not significantly inhibit human Cytochrome P450 enzymes, leading to fewer drug-drug interactions. * **Mnemonic for Ergosterol Synthesis Inhibitors:** **S**qualene → (**A**llylamines) → **L**anosterol → (**A**zoles) → **E**rgosterol. (Think: **S**top **A**t **L**anosterol **A**nd **E**nd).

Question 77: What are the common natural flora of the skin?

A. Streptococcus
B. Staphylococcus aureus
C. Candida albicans
D. All of the above (Correct Answer)

Explanation: **Explanation:** The human skin is a complex ecosystem harboring a diverse population of microorganisms known as the **normal flora**. These organisms are categorized into **resident flora** (permanent inhabitants) and **transient flora** (temporary colonizers). **Why 'All of the above' is correct:** The skin microbiome is dominated by Gram-positive bacteria and certain fungi that can withstand the skin's dry, acidic, and salty environment. * **Streptococcus:** Various species (especially viridans group) are found as transient or resident flora, particularly in moist areas. * **Staphylococcus aureus:** While *Staphylococcus epidermidis* (CoNS) is the most abundant resident, *S. aureus* is a common transient colonizer, found in the nostrils and intertriginous areas of roughly 20-30% of healthy individuals. * **Candida albicans:** This fungus is a normal commensal of the skin, gastrointestinal tract, and vagina. It typically remains non-pathogenic unless the skin barrier is breached or the host is immunocompromised. **Analysis of Options:** * **Option A & B:** Gram-positive cocci like *Staphylococcus* and *Streptococcus* are the primary bacterial constituents. *Propionibacterium acnes* (now *Cutibacterium*) is another major inhabitant of sebaceous glands. * **Option C:** *Candida* and *Malassezia furfur* are the most significant fungal components of the skin's natural flora. **High-Yield Clinical Pearls for NEET-PG:** 1. **Most common resident:** *Staphylococcus epidermidis* is the most ubiquitous organism on the skin. 2. **Dominant Anaerobe:** *Cutibacterium acnes* resides in hair follicles and is implicated in acne vulgaris. 3. **Surgical Importance:** Pre-operative skin antisepsis (e.g., Povidone-iodine or Chlorhexidine) aims to reduce these flora to prevent **Surgical Site Infections (SSIs)**. 4. **Nasal Carriage:** The anterior nares are the primary reservoir for *Staphylococcus aureus*.

Question 78: Which beta-hemolytic Streptococcus is commonly implicated in neonatal meningitis?

A. Group A
B. Group B (Correct Answer)
C. Group C
D. Group D

Explanation: **Explanation:** The correct answer is **Group B Streptococcus (GBS)**, specifically *Streptococcus agalactiae*. **Why Group B is correct:** *Streptococcus agalactiae* is a Gram-positive, beta-hemolytic coccus that colonizes the maternal gastrointestinal and vaginal tracts in approximately 10–30% of pregnant women. During childbirth, the neonate can acquire the bacteria via vertical transmission. GBS is the **most common cause** of both early-onset (0–6 days) and late-onset (7–89 days) neonatal sepsis and meningitis. Its primary virulence factor is the polysaccharide capsule, which inhibits phagocytosis. **Why the other options are incorrect:** * **Group A (S. pyogenes):** Primarily causes pharyngitis, pyoderma, and immune-mediated sequelae like Rheumatic Fever. While it is beta-hemolytic, it is rarely a cause of neonatal meningitis. * **Group C:** These (e.g., *S. dysgalactiae*) can cause pharyngitis or skin infections but are uncommon human pathogens and do not typically cause neonatal disease. * **Group D:** This group includes *Enterococci* and *S. bovis*. While *Enterococcus* can cause neonatal sepsis, it is typically **gamma-hemolytic** (non-hemolytic) or alpha-hemolytic, not beta-hemolytic. **High-Yield Clinical Pearls for NEET-PG:** * **CAMP Test:** GBS produces the "CAMP factor," which enhances the beta-hemolysis of *Staphylococcus aureus* (forming an arrow-shaped zone). * **Bacitracin Sensitivity:** GBS is **Bacitracin resistant**, whereas Group A Strep is Bacitracin sensitive. * **Prevention:** Screening pregnant women at 36–37 weeks gestation and administering intrapartum antibiotic prophylaxis (usually Penicillin G) significantly reduces the risk of early-onset disease. * **Other Neonatal Meningitis Pathogens:** Remember the "Big Three": *Group B Strep*, *E. coli* (K1 strain), and *Listeria monocytogenes*.

Question 79: Paul-Bunnel test is done for which of the following?

A. Hepatitis B Virus (HBV)
B. Epstein-Barr Virus (EBV) (Correct Answer)
C. Cytomegalovirus (CMV)
D. Human Immunodeficiency Virus (HIV)

Explanation: **Explanation:** The **Paul-Bunnell test** is a classic diagnostic tool used for **Infectious Mononucleosis (IM)**, primarily caused by the **Epstein-Barr Virus (EBV)**. **1. Why EBV is correct:** EBV infects B-lymphocytes, leading to their polyclonal activation. This results in the production of **Heterophile antibodies**—IgM antibodies that do not react with EBV antigens themselves but have the unique property of agglutinating red blood cells (RBCs) from other species (sheep, horse, or ox). The Paul-Bunnell test specifically detects these heterophile antibodies by observing the agglutination of **sheep RBCs**. **2. Why other options are incorrect:** * **Hepatitis B Virus (HBV):** Diagnosis relies on specific serological markers (HBsAg, anti-HBc, HBeAg) and molecular testing (HBV DNA), not heterophile antibodies. * **Cytomegalovirus (CMV):** While CMV causes a clinical syndrome similar to IM (fever, lymphadenopathy), it is characteristically **Heterophile-negative**. This is a key differential point in exams. * **HIV:** Diagnosis is made via p24 antigen assays, ELISA for antibodies, and Western Blot or PCR for confirmation. **NEET-PG High-Yield Pearls:** * **Monospot Test:** A modern, rapid version of the Paul-Bunnell test using horse RBCs. * **Differential Absorption (Davidsohn) Test:** Used to distinguish EBV heterophile antibodies from those found in Serum Sickness or Forssman antibodies using guinea pig kidney cells and beef RBCs. * **Atypical Lymphocytes (Downey Cells):** These are activated T-cells (CD8+) seen on a peripheral smear in EBV infection. * **False Negatives:** The Paul-Bunnell test is often negative in children under 5 years of age and during the first week of illness.

Question 80: A neonate develops signs of meningitis at seven days of birth. The presence of which of the following infectious agents in the maternal genital tract can be the causative agent of this disease?

A. Neisseria gonorrhea
B. Chlamydia trachomatis
C. Streptococcus agalactiae (Correct Answer)
D. Haemophilus ducreyi

Explanation: ### Explanation **1. Why Streptococcus agalactiae is correct:** *Streptococcus agalactiae*, also known as **Group B Streptococcus (GBS)**, is the leading cause of neonatal sepsis and meningitis. It is a normal commensal of the maternal gastrointestinal and genitourinary tracts (found in ~25% of pregnant women). The neonate acquires the infection during passage through the birth canal or via ascending infection. * **Early-onset disease** (0–7 days) typically presents as pneumonia or sepsis. * **Late-onset disease** (7 days to 3 months) frequently presents as **meningitis**. **2. Why the other options are incorrect:** * **Neisseria gonorrhoeae:** Primarily causes **Ophthalmia neonatorum** (purulent conjunctivitis) in newborns, not meningitis. * **Chlamydia trachomatis:** Causes inclusion conjunctivitis and **interstitial pneumonia** (characteristically presenting with a "staccato cough") in neonates, but is not a common cause of meningitis. * **Haemophilus ducreyi:** This is the causative agent of **Chancroid** (painful genital ulcers). It is not part of the normal vaginal flora and does not cause neonatal meningitis. **3. Clinical Pearls for NEET-PG:** * **Top 3 causes of Neonatal Meningitis:** 1. *Streptococcus agalactiae* (GBS), 2. *Escherichia coli* (K1 antigen), 3. *Listeria monocytogenes*. * **Screening:** Pregnant women are screened for GBS colonization at **35–37 weeks** of gestation. * **Prophylaxis:** If the mother is GBS-positive, intrapartum antibiotic prophylaxis (usually **IV Penicillin G**) is administered to prevent transmission. * **Lab Diagnosis:** GBS is Gram-positive, catalase-negative, and shows **CAMP test positivity** (enhanced hemolysis with *S. aureus*).

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