Diagnostic Microbiology Practice Questions

Q: All of the following bacteria test "Urease positive" except:

E. coli. **Explanation:** The urease test identifies organisms capable of producing the enzyme **urease**, which hydrolyzes urea into ammonia and carbon dioxide. The production of ammonia raises the pH of the medium, changing the indicator (usually phenol red) from yellow to pink/magenta. 1. **Why E. coli is the correct answer:** * *Escherichia coli* is a classic **urease-negative** organism. In the laboratory, this is a key biochemical feature used to differentiate it from other members of the Enterobacteriaceae family, such as *Proteus* or *Klebsiella*. 2. **Analysis of Incorrect Options:** * **Proteus (Option B):** This is the most famous urease-positive genus. It produces urease in large quantities ("rapid urease positive"), which is a major virulence factor in causing urinary tract infections (UTIs). * **Klebsiella (Option C):** Most species of *Klebsiella* (e.g., *K. pneumoniae*) are urease-positive, though they typically react more slowly than *Proteus*. * **Staphylococcus (Option D):** *Staphylococcus saprophyticus* and *Staphylococcus epidermidis* are notable urease producers. *S. saprophyticus* uses urease to colonize the urinary tract in young women. **High-Yield Clinical Pearls for NEET-PG:** * **Struvite Stones:** Urease-positive bacteria (especially *Proteus*) increase urinary pH, leading to the precipitation of magnesium ammonium phosphate, forming **Staghorn calculi** (Struvite/Triple phosphate stones). * **Mnemonic for Urease-Positive Organisms (PUNCH):** **P**roteus, **U**reaplasma, **N**ocardia, **C**ryptococcus, **H**elicobacter pylori. (Others include *Klebsiella*, *S. saprophyticus*, and *Brucella*). * **H. pylori:** Uses urease to neutralize gastric acid, allowing it to survive in the stomach. This is the basis for the **Urea Breath Test**.

Q: Diagnosis of enteric fever in the 2nd week is best made by which method?

Widal test. To master the diagnosis of Enteric Fever (caused by *Salmonella Typhi*), it is essential to remember the **"BASU"** mnemonic, which dictates the sequence of positive diagnostic tests over the course of the illness. ### **Explanation of the Correct Answer** **B. Widal Test:** This is a serological test that detects antibodies (Anti-O and Anti-H) against *Salmonella* antigens. These antibodies typically appear in the serum at the end of the first week, but their titers reach a diagnostically significant level during the **2nd week** of fever. Therefore, it is the investigation of choice for this specific timeframe. ### **Analysis of Incorrect Options** * **A. Blood Culture:** This is the gold standard for diagnosis in the **1st week** of illness (positive in ~90% of cases). By the 2nd week, the bacteremia begins to subside as the bacteria localize in lymphoid tissues, reducing the sensitivity of blood cultures. * **C. Stool Culture:** Bacteria are shed in the feces after being excreted from the gallbladder. This test becomes most reliable during the **3rd week** of infection. * **D. Urine Culture:** This is typically the last to become positive, usually during the **4th week** of the disease course. ### **NEET-PG High-Yield Pearls** * **BASU Mnemonic:** **B**lood (1st week), **A**gglutination/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). * **Bone Marrow Culture:** This is the **most sensitive** method overall and remains positive even if the patient has already started antibiotics. * **Widal Interpretation:** A four-fold rise in titers between paired sera (taken 7-10 days apart) is more significant than a single reading. * **Castaneda’s Medium:** A biphasic medium often used for blood cultures to reduce the risk of contamination during subculturing.

Q: Which of the following investigations is NOT typically used for the diagnosis of Tuberculosis?

Albumin levels. **Explanation:** The diagnosis of Tuberculosis (TB) relies on the identification of the pathogen (*Mycobacterium tuberculosis*), its genetic material, or the characteristic host immune response. **Why Albumin levels are the correct answer:** Albumin is a negative acute-phase reactant synthesized by the liver. While chronic infections like TB can lead to hypoalbuminemia due to malnutrition or chronic inflammation, **albumin levels are non-specific** and hold no diagnostic value for confirming TB. They are used to assess nutritional status rather than to diagnose the infection itself. **Analysis of incorrect options:** * **GeneXpert (CBNAAT):** This is the **initial diagnostic test of choice** according to RNTCP/NTEP guidelines. It is a molecular assay that detects TB DNA and rifampicin resistance simultaneously within 2 hours. * **Adenosine Deaminase (ADA) levels:** ADA is an enzyme released by T-lymphocytes during activation. Elevated ADA levels in pleural, peritoneal, or cerebrospinal fluid are highly suggestive of **Extrapulmonary TB** (e.g., TB pleuritis or meningitis). * **Lymphocyte counts:** TB is a chronic granulomatous inflammation characterized by a **Type IV hypersensitivity reaction**. A high lymphocyte count (lymphocytosis) in fluid analysis (e.g., pleural fluid or CSF) is a classic diagnostic clue for TB. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** Culture on **LJ Medium** (takes 6–8 weeks) or Liquid Culture (**MGIT** - takes 1–3 weeks). * **Z-N Staining:** Requires at least $10^4$ bacilli/ml for positivity. * **ADA Cut-off:** Typically >40 U/L in pleural fluid is highly suggestive of TB. * **GeneXpert Ultra:** A more sensitive version of GeneXpert, useful in paucibacillary cases (smear-negative TB).

Q: Which of the following cannot be detected by wet film microscopy?

Chlamydia. ### Explanation The correct answer is **Chlamydia (Option C)**. **1. Why Chlamydia is the Correct Answer:** *Chlamydia trachomatis* is an **obligate intracellular bacterium**. Because of its extremely small size (0.2–0.5 μm) and the fact that it resides within host cells, it cannot be visualized using standard light microscopy or wet mount preparations. Diagnosis typically requires **Nucleic Acid Amplification Tests (NAAT)**, which is the gold standard, or direct fluorescent antibody (DFA) staining and cell culture. **2. Analysis of Incorrect Options:** * **Candida (Option A):** A wet mount (often with 10% KOH) easily identifies budding yeast cells and **pseudohyphae**, which are characteristic of vaginal candidiasis. * **Trichomonas (Option B):** Saline wet film is the classic diagnostic method for *Trichomonas vaginalis*. It reveals pear-shaped, flagellated protozoa showing characteristic **jerky, twitching motility**. * **Bacterial Vaginosis (Option D):** Diagnosis is made using **Amsel’s criteria**, one of which is the presence of **"Clue Cells"** (vaginal epithelial cells coated with *Gardnerella vaginalis*) on a saline wet mount. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Chlamydia:** NAAT (Nucleic Acid Amplification Test). * **Whiff Test:** Adding 10% KOH to vaginal discharge; a "fishy odor" indicates Bacterial Vaginosis (due to release of amines). * **pH Changes:** Vaginal pH is **>4.5** in Trichomoniasis and Bacterial Vaginosis, but remains **normal (4.0–4.5)** in Candidiasis. * **Strawberry Cervix:** A classic clinical sign associated with *Trichomonas vaginalis*.

Q: ASO titre is useful in the diagnosis of which of the following?

S. pyogenes. ### Explanation **Correct Answer: B. S. pyogenes** **Why it is correct:** The **Anti-Streptolysin O (ASO) titre** is a serological test used to detect a recent infection with **Group A Streptococcus (GAS)**, specifically *Streptococcus pyogenes*. *S. pyogenes* produces an oxygen-labile exotoxin called **Streptolysin O**, which is highly antigenic. In response, the body produces ASO antibodies. A rise in these titres is clinically significant for diagnosing **nonsuppurative post-streptococcal complications**, such as **Acute Rheumatic Fever (ARF)** and **Post-Streptococcal Acute Glomerulonephritis (PSAGN)**. **Why the other options are incorrect:** * **A. S. bovis:** Now reclassified as *S. gallolyticus* (Group D), it is primarily associated with endocarditis and colorectal cancer. It does not produce Streptolysin O. * **C. S. agalactiae:** Known as Group B Streptococcus (GBS), it is a leading cause of neonatal sepsis and meningitis. It lacks the Streptolysin O toxin. * **D. S. pneumoniae:** While it produces a similar toxin called *Pneumolysin*, it does not produce Streptolysin O; therefore, the ASO test is not used for its diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **ASO Cut-off:** A titre of **>200 units** is generally considered significant in adults. * **The "Skin" Exception:** ASO titres are typically **elevated in post-streptococcal pharyngitis** but often **remain low or absent in streptococcal skin infections** (Impetigo/Pyoderma) because skin cholesterol inhibits Streptolysin O. For skin infections, the **Anti-DNase B** test is the preferred diagnostic marker. * **Todd Units:** ASO titres are traditionally expressed in Todd units. * **Clinical Utility:** ASO is used to confirm *past* infection; it is not useful for diagnosing acute pharyngitis.

Q: In which of the following is a throat swab best transported to the lab?

Plate with transport medium. **Explanation:** The primary goal of transporting a clinical specimen is to maintain the viability of potential pathogens while preventing the overgrowth of contaminating flora. For a throat swab, the correct method is using a **plate or tube containing a transport medium** (Option B). **Why Option B is correct:** Throat swabs are frequently used to diagnose infections like *Streptococcus pyogenes* (Group A Strep). Many respiratory pathogens are fastidious and highly sensitive to drying (desiccation) and changes in pH. Transport media (such as **Pike’s medium** or **Amies/Stuart’s medium**) contain buffering agents and lack high-nutrient substrates to prevent commensal overgrowth, ensuring the pathogen survives the transit time to the laboratory. **Why other options are incorrect:** * **Options A & C (Plastic/Test tubes):** If these containers are empty (dry), the swab will dry out rapidly. Desiccation is lethal to many delicate bacteria, leading to false-negative culture results. * **Option D (No covering):** Leaving a swab uncovered leads to immediate environmental contamination and rapid drying, making it clinically useless. **NEET-PG High-Yield Pearls:** * **Pike’s Medium:** Specifically used for transporting *Streptococcus pyogenes* from throat swabs; it contains blood and crystal violet to inhibit skin contaminants. * **Stuart’s/Amies Medium:** Common non-nutritive transport media used for various swabs (including throat and urogenital). * **Viral Transport Medium (VTM):** If the throat swab is for viral pathogens (e.g., Influenza or SARS-CoV-2), it must be placed in VTM containing proteins (albumin/gelatin) and antibiotics to inhibit bacterial growth. * **Diphtheria:** If *Corynebacterium diphtheriae* is suspected, the swab should ideally be inoculated directly onto **Loeffler’s Serum Slope** for rapid growth.

Q: What is the common stain used for fungal hyphae?

Gomori Methanamine silver. **Explanation:** **Gomori Methenamine Silver (GMS)** is considered the "gold standard" and most commonly used special stain for the histological identification of fungal elements. The underlying principle is an **oxidation-reduction reaction**: chromic acid oxidizes the polysaccharides (glucans and chitin) in the fungal cell wall to form aldehydes. These aldehydes then reduce the silver ions in the methenamine silver complex to metallic silver, staining the hyphae and spores **black or dark brown** against a light green background. **Analysis of Incorrect Options:** * **Methylene blue:** Primarily used as a simple bacterial stain or as a counterstain (e.g., in Ziehl-Neelsen). While Lactophenol Cotton Blue (LPCB) is used for fungal wet mounts, plain Methylene blue is not a standard stain for hyphae in tissue. * **Congo red:** Specifically used to identify **Amyloid** fibrils (showing apple-green birefringence). While it can bind to chitin, it is not the standard diagnostic stain for fungi. * **Oil red O:** A lipid-soluble dye used to demonstrate **neutral triglycerides and lipids** in frozen sections; it has no affinity for fungal cell walls. **High-Yield Clinical Pearls for NEET-PG:** * **PAS (Periodic Acid-Schiff):** Another vital fungal stain; it stains fungal walls **bright pink/magenta**. * **Mucicarmine:** Specifically used to identify the capsule of ***Cryptococcus neoformans*** (stains it red). * **India Ink:** Used for negative staining of *Cryptococcus* in CSF. * **Calcofluor White:** A fluorescent stain that binds to chitin; it is the most sensitive rapid method for detecting fungi in clinical specimens.

Question 1

All of the following bacteria test "Urease positive" except:

Accepted Answer

E. coli

Answer

Proteus

Answer

Klebsiella

Answer

Staphylococcus

Question 2

Diagnosis of enteric fever in the 2nd week is best made by which method?

Accepted Answer

Widal test

Answer

Blood culture

Answer

Stool culture

Answer

Urine culture

Question 3

Which of the following investigations is done to diagnose typhoid in a patient after 15 days of onset of fever?

Accepted Answer

Widal test

Answer

Blood culture

Answer

Stool culture

Answer

Urine culture

Question 4

Which of the following investigations is NOT typically used for the diagnosis of Tuberculosis?

Accepted Answer

Albumin levels

Answer

GeneXpert assay

Answer

Adenosine Deaminase levels

Answer

Lymphocyte counts

Question 5

A 15-year-old male presents with fever (103°F), severe headache, stiff neck, and a petechial rash. A lumbar puncture reveals Gram-negative diplococci (Neisseria meningitidis). Which of the following tests, combining features of gel diffusion and immunoelectrophoresis, is applicable to negatively charged antigens for confirming the causative agent?

Accepted Answer

Counter immunoelectrophoresis (CIE)

Answer

Coagglutination (COA)

Answer

Enzyme-linked immunosorbent assay (ELISA)

Answer

Latex agglutination (LA)

Question 6

A 2-year-old child presents with fever, stiff neck, and irritability. A Gram stain smear of cerebrospinal fluid reveals small, pleomorphic, gram-negative coccobacillary organisms. What is the most appropriate procedure to establish an etiological diagnosis?

Accepted Answer

Culture the cerebrospinal fluid on chocolate agar and identify the organism by growth factors.

Answer

Culture the cerebrospinal fluid on mannitol-salt agar.

Answer

Perform a catalase test on the isolated organism.

Answer

Perform a coagulase test on the isolated organism.

Question 7

Which of the following cannot be detected by wet film microscopy?

Accepted Answer

Chlamydia

Answer

Candida

Answer

Trichomonas

Answer

Bacterial vaginosis

Question 8

ASO titre is useful in the diagnosis of which of the following?

Accepted Answer

S. pyogenes

Answer

S. bovis

Answer

S. agalactiae

Answer

S. pneumonia

Question 9

In which of the following is a throat swab best transported to the lab?

Accepted Answer

Plate with transport medium

Answer

Plastic tube

Answer

Test tube

Answer

No covering

Question 10

What is the common stain used for fungal hyphae?

Accepted Answer

Gomori Methanamine silver

Answer

Methylene blue

Answer

Congo red

Answer

Oil red O

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?