Diagnostic Microbiology Practice Questions

Q: Tzank smear is used for the diagnosis of which of the following?

Herpes Simplex Virus (HSV). **Explanation:** The **Tzanck smear** is a rapid bedside diagnostic test used primarily for the presumptive diagnosis of infections caused by the **Herpesviridae** family, specifically **Herpes Simplex Virus (HSV-1, HSV-2)** and **Varicella-Zoster Virus (VZV)**. **Why the correct answer is right:** The procedure involves scraping the base of a fresh vesicular lesion and staining it with Giemsa, Wright, or Leishman stain. The hallmark finding is the presence of **multinucleated giant cells** (formed by the fusion of infected keratinocytes) and **Acantholysis** (loss of intercellular connections). It may also show eosinophilic intranuclear inclusion bodies known as **Cowdry Type A bodies**. **Analysis of Incorrect Options:** * **B. Psittacosis:** Caused by *Chlamydia psittaci*. Diagnosis is typically made via serology (CFT) or PCR. Microscopic examination would show **LCL bodies** (Levinthal-Cole-Lillie), not Tzanck cells. * **C. Cryptococcus:** This is a fungus diagnosed using **India Ink** preparation (to see the capsule) or Mucicarmine stain. * **D. Rickettsia:** These are obligate intracellular bacteria diagnosed via the **Weil-Felix reaction** (serology) or Gimenez stain. **High-Yield Clinical Pearls for NEET-PG:** * **Limitation:** Tzanck smear **cannot distinguish** between HSV-1, HSV-2, and VZV. For definitive differentiation, viral culture or PCR is required. * **Other uses:** Apart from Herpes, Tzanck smear is used in dermatology to diagnose **Pemphigus Vulgaris** (shows Tzanck cells/acantholytic cells) and **Cytomegalovirus (CMV)**. * **Don't confuse:** Cowdry Type A (Herpes) vs. Cowdry Type B (Polio/Adenovirus).

Q: Leishmania is cultured in which medium?

NNN. **Explanation:** **1. Why NNN Medium is Correct:** *Leishmania* species (the causative agents of Kala-azar and Cutaneous Leishmaniasis) are traditionally cultured in **NNN (Novy-MacNeal-Nicolle) medium**. This is a specialized **biphasic medium** consisting of a solid phase (blood agar made with rabbit blood) and a liquid phase (overlay of saline or broth). In this medium, the parasite transforms from the amastigote form (found in humans) into the **flagellated promastigote form**, which multiplies in the liquid phase. **2. Why Other Options are Incorrect:** * **Chocolate Agar:** This is an enriched medium used for fastidious bacteria like *Haemophilus influenzae* and *Neisseria meningitidis*. It is essentially blood agar where the red cells have been lysed by heat. * **Tellurite (Potassium Tellurite Agar):** This is a selective medium used for **Corynebacterium diphtheriae**. The bacteria reduce tellurite to metallic tellurium, resulting in characteristic black-colored colonies. * **Sabouraud’s Dextrose Agar (SDA):** This is the standard medium used for the cultivation of **Fungi** (molds and yeasts). Its low pH inhibits bacterial growth. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** While NNN culture is highly specific, the gold standard for Kala-azar diagnosis remains the demonstration of **LD bodies (Amastigotes)** in splenic or bone marrow aspirates. * **Other Media for Leishmania:** Apart from NNN, **Schneider’s Drosophila medium** (liquid medium) is also used. * **Vector:** *Leishmania donovani* is transmitted by the bite of the female **Sandfly (*Phlebotomus argentipes*)**. * **Drug of Choice:** Liposomal Amphotericin B is currently the preferred treatment for Visceral Leishmaniasis.

Q: What is the best specimen for rabies diagnosis in a living person?

Biopsy of hair follicle from the neck. **Explanation:** The diagnosis of rabies in a living patient (antemortem) relies on detecting the virus in tissues with high nerve density or active viral shedding. **Why Option C is Correct:** The **full-thickness skin biopsy from the nape of the neck** is considered the gold standard for antemortem diagnosis. Rabies virus is highly neurotropic; it travels via cutaneous nerves to the base of hair follicles. Using **Direct Fluorescent Antibody (DFA)** testing or RT-PCR on this biopsy specimen offers high sensitivity and specificity. The nape of the neck is preferred because it has a high density of hair follicles and sensory nerves. **Analysis of Incorrect Options:** * **A. Corneal smear:** While historically used, it is no longer preferred due to low sensitivity and the risk of eye injury during collection. * **B. Cerebrospinal fluid (CSF):** Viral RNA or antibodies are rarely detected in CSF early in the disease. It is more useful for ruling out other types of encephalitis than confirming rabies. * **D. Brain biopsy:** While the brain is the definitive site for diagnosis (looking for **Negri bodies**), it is an invasive procedure and is generally reserved for post-mortem examination. **NEET-PG High-Yield Pearls:** * **Post-mortem Gold Standard:** DFA on brain tissue (specifically the Hippocampus/Ammon’s horn and Cerebellum). * **Negri Bodies:** Intracytoplasmic, eosinophilic inclusion bodies found in neurons (absent in 20-30% of cases). * **Other Antemortem Samples:** Saliva (for RT-PCR) and Serum (for antibodies in unvaccinated individuals). * **Rule of Thumb:** No single test is 100% sensitive; a combination of skin biopsy, saliva, and serum is often used clinically.

Q: A young boy developed fever and axillary lymphadenopathy 5 days after a flea bite, likely acquired while working in a wheat grain godown. Which one of the following staining methods would help in the identification of the suspected pathogen?

Wayson staining. ### Explanation **Correct Option: D. Wayson staining** The clinical presentation of fever and axillary lymphadenopathy (bubo) following a flea bite in a grain godown (where rodents/rats are common) strongly suggests **Bubonic Plague**, caused by ***Yersinia pestis***. * **Underlying Concept:** *Yersinia pestis* is a Gram-negative coccobacillus that exhibits a characteristic **"safety-pin" appearance** (bipolar staining). This occurs because the ends of the bacilli stain more intensely than the center. * **Wayson stain** (a modification of methylene blue and carbol fuchsin) is the preferred rapid diagnostic method to demonstrate this bipolar staining. Giemsa and Wright stains can also be used for this purpose. **Analysis of Incorrect Options:** * **A. Albert staining:** Used for the demonstration of metachromatic granules (Volutin granules) in ***Corynebacterium diphtheriae***. * **B. Ziehl-Neelsen staining:** An acid-fast stain used primarily for ***Mycobacterium tuberculosis*** and *Mycobacterium leprae*. * **C. McFadyean's staining:** A polychrome methylene blue stain used to demonstrate the capsule of ***Bacillus anthracis*** (Anthrax), showing a characteristic "M'Fadyean reaction" (pinkish-purple capsular material around blue bacilli). **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** The Oriental rat flea (*Xenopsylla cheopis*). * **Reservoir:** Wild rodents (sylvatic plague) and urban rats (urban plague). * **Morphology:** *Y. pestis* is a non-motile, non-sporing, Gram-negative bacillus. It is **catalase positive** and **oxidase negative**. * **Culture:** Shows "Ghee-like" growth in broth and "Stalactite" growth. On agar, it forms "Fried egg" colonies. * **Drug of Choice:** Streptomycin or Gentamicin (Tetracyclines/Doxycycline are alternatives).

Q: What stain is employed for Mycoplasma?

Giemsa stain. **Explanation:** **Mycoplasma** are unique bacteria characterized by the **complete absence of a peptidoglycan cell wall**. Instead, their cell membrane contains sterols. Because they lack a cell wall, they do not take up Gram stain and are highly pleomorphic. 1. **Why Giemsa Stain is Correct:** Since Mycoplasma cannot be visualized by Gram staining, they require specialized staining techniques. **Giemsa stain** is the preferred method as it effectively stains the DNA and cytoplasmic components of these minute organisms, making them appear as tiny, pale pink or purple coccobacillary bodies under a light microscope. Diene’s stain is another specific stain used for visualizing Mycoplasma colonies on agar. 2. **Why Other Options are Incorrect:** * **Acid-fast stain:** Used for *Mycobacterium* species (like *M. tuberculosis*) which have high mycolic acid content in their cell walls. Mycoplasma lacks a cell wall entirely. * **Methylene blue stain:** While a simple stain, it lacks the contrast and specificity provided by Giemsa for identifying the delicate structure of Mycoplasma. * **Congo red stain:** Primarily used in pathology to identify **Amyloid** fibrils (showing apple-green birefringence), not for bacterial identification. **High-Yield Clinical Pearls for NEET-PG:** * **Culture:** Mycoplasma grows on **PPLO agar** (Pleuropneumonia-like organisms). * **Colony Morphology:** They produce characteristic **"Fried Egg" colonies** (central zone grows into the agar). * **Antibiotic Resistance:** They are **innately resistant to Beta-lactams** (Penicillins/Cephalosporins) because they lack a cell wall target. Macrolides (Azithromycin) or Tetracyclines are the drugs of choice. * **Eaton’s Agent:** *Mycoplasma pneumoniae* is the most common cause of **Atypical Pneumonia** (Walking Pneumonia).

Q: What are the confirmatory tests for Corynebacterium diphtheriae?

Isolation in selective media. **Explanation:** The laboratory diagnosis of *Corynebacterium diphtheriae* follows a specific hierarchy. While clinical suspicion is paramount, the **confirmatory test** for identifying the organism itself is its **isolation in selective media**. 1. **Why "Isolation in selective media" is correct:** To definitively confirm the presence of *C. diphtheriae*, the organism must be grown from a throat or nasopharyngeal swab. Selective media like **Hoyle’s Tellurite Agar** or **Potassium Tellurite Agar** are used to inhibit normal flora and allow *C. diphtheriae* to grow as characteristic grey-black colonies (due to tellurite reduction). **Loeffler’s Serum Slope** is used for rapid growth (6-8 hours) and to enhance the development of metachromatic granules. 2. **Why other options are incorrect:** * **Tests for toxin production (e.g., Elek’s test):** These are used to determine the **virulence (toxigenicity)** of the isolated strain, not to identify the species itself. While crucial for clinical management, isolation must precede toxigenicity testing. * **Serological tests:** These are not used for the routine diagnosis of acute diphtheria as they do not provide timely or specific confirmation of an active infection. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-motile, club-shaped bacilli arranged in "Chinese letter" or cuneiform patterns. * **Granules:** **Volutin/Babes-Ernst granules** (metachromatic) are visualized using **Albert’s stain**. * **Toxigenicity Testing:** **Elek’s Gel Precipitation Test** (in vitro) and the **Schick test** (in vivo skin test to determine immunity status, though now largely historical). * **Mechanism:** Diphtheria toxin inhibits protein synthesis by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Q: When should a urine sample be collected from a patient with renal tuberculosis?

Early morning urine collection. **Explanation:** The diagnosis of renal tuberculosis (TB) relies on the detection of *Mycobacterium tuberculosis* in the urine. The correct answer is **Early morning urine collection** because tubercle bacilli are shed intermittently and are often present in low numbers. 1. **Why Option C is correct:** Overnight, urine remains in the bladder for an extended period, allowing for the concentration of bacilli. An early morning, mid-stream sample (collected on three to five consecutive days) provides the highest diagnostic yield for both microscopy (AFB staining) and culture (MGIT/LJ medium). 2. **Why Options A and B are incorrect:** Historically, 24-hour or 12-hour pooled urine samples were used. However, these are no longer recommended because: * The acidic environment of pooled urine for long durations is **toxic to the bacilli**, reducing their viability for culture. * Pooled samples are highly prone to **commensal contamination** (e.g., *Mycobacterium smegmatis*), making interpretation difficult. 3. **Why Option D is incorrect:** Random samples are likely to be too dilute, significantly increasing the risk of a false-negative result. **High-Yield Clinical Pearls for NEET-PG:** * **Sterile Pyuria:** The classic presentation of renal TB is "Sterile Pyuria"—the presence of pus cells (WBCs) in the urine without the growth of common pyogenic bacteria on routine culture media. * **Gold Standard:** Culture on **Lowenstein-Jensen (LJ) medium** or liquid systems like **MGIT** remains the gold standard, though GeneXpert (NAAT) is increasingly used for rapid detection. * **Sample Requirement:** At least **three consecutive** early morning samples are required to achieve maximum sensitivity.

Q: Which of the following is a true statement regarding the Widal test in typhoid fever?

Previous infection alters Widal test results.. ### Explanation **Correct Option: C. Previous infection alters Widal test results.** The Widal test is a tube agglutination test that detects antibodies (anti-O and anti-H) against *Salmonella Typhi* and *Paratyphi*. In individuals who have had a previous infection, **anamnestic responses** occur. Memory B-cells can cause a rapid rise in antibody titers during any subsequent unrelated febrile illness, leading to a false-positive result. Therefore, a single positive Widal test is often unreliable for diagnosing an acute infection. **Analysis of Incorrect Options:** * **A. The Widal test is confirmative in endemic areas:** In endemic areas, a significant portion of the healthy population may carry baseline antibodies. Thus, a single test is never confirmative; only a **four-fold rise** in titers in paired sera (collected 7–10 days apart) is diagnostic. * **B. Antibiotic treatment does not alter Widal test results:** Early administration of antibiotics (like Chloramphenicol or Ciprofloxacin) can suppress the immune response, leading to low or absent antibody titers, potentially causing a false-negative result. * **D. The Widal test is not altered by prior vaccination:** Immunization with the TAB vaccine or older parenteral typhoid vaccines induces high levels of H and O antibodies, which can persist for months, interfering with the interpretation of the test. **High-Yield Clinical Pearls for NEET-PG:** * **Timing:** Widal becomes positive only after the **first week** of fever (maximal sensitivity in the 2nd–3rd week). * **Diagnostic Titers:** Generally, **O > 1:80** and **H > 1:160** are considered significant, but this varies by local endemicity. * **Gold Standard:** Bone marrow culture is the most sensitive; Blood culture is the investigation of choice in the 1st week (**BASU** mnemonic: **B**lood-1st week, **A**ntibody/Widal-2nd, **S**tool-3rd, **U**rine-4th).

Question 1

Tzank smear is used for the diagnosis of which of the following?

Accepted Answer

Herpes Simplex Virus (HSV)

Answer

Psittacosis

Answer

Cryptococcus

Answer

Rickettsia

Question 2

Leishmania is cultured in which medium?

Accepted Answer

NNN

Answer

Chocolate agar

Answer

Tellurite

Answer

Sabouraud's agar

Question 3

What is the best specimen for rabies diagnosis in a living person?

Accepted Answer

Biopsy of hair follicle from the neck

Answer

Corneal smear

Answer

Cerebrospinal fluid (CSF)

Answer

Brain biopsy

Question 4

A young boy developed fever and axillary lymphadenopathy 5 days after a flea bite, likely acquired while working in a wheat grain godown. Which one of the following staining methods would help in the identification of the suspected pathogen?

Accepted Answer

Wayson staining

Answer

Albert staining

Answer

Ziehl-Neelsen staining

Answer

McFadyean's staining

Question 5

What stain is employed for Mycoplasma?

Accepted Answer

Giemsa stain

Answer

Acid fast stain

Answer

Methylene blue stain

Answer

Congo red stain

Question 6

What are the confirmatory tests for Corynebacterium diphtheriae?

Accepted Answer

Isolation in selective media

Answer

Serological tests

Answer

Tests for toxin production

Answer

All of the above

Question 7

Which of the following is NOT a method for the isolation of Chlamydia from clinical specimens?

Accepted Answer

Enzyme immunoassay

Answer

Yolk sac inoculation

Answer

Tissue culture using irradiated McCoy cells

Answer

Tissue culture using irradiated BHK cells

Question 8

When should a urine sample be collected from a patient with renal tuberculosis?

Accepted Answer

Early morning urine collection

Answer

24 hours urine collection

Answer

12 hours urine collection

Answer

Urine collection at any time

Question 9

Regarding syphilis, all of the following statements are true, except?

Accepted Answer

VDRL titre decreases or becomes absent after treatment

Answer

VDRL becomes positive 10-14 days after infection

Answer

The earliest serological test to become positive in untreated primary syphilis is TPHA

Answer

Yaws and Pinta cannot be differentiated from syphilis serologically

Question 10

Which of the following is a true statement regarding the Widal test in typhoid fever?

Accepted Answer

Previous infection alters Widal test results.

Answer

The Widal test is confirmative in endemic areas.

Answer

Antibiotic treatment does not alter Widal test results.

Answer

The Widal test is not altered by prior vaccination.

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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