INI-CET 2025 - Microbiology Practice Questions

Q: Identify the antibody shown in the diagram which has a joining chain and is mainly found in mucosal areas.

IgA. ***IgA*** - The diagram shows a **dimeric** antibody structure, which consists of two monomer units linked by a **J chain** (joining chain) and a **secretory component**, a classic representation of secretory IgA (sIgA). - IgA is the predominant immunoglobulin found in **mucosal secretions** such as saliva, tears, breast milk, and gastrointestinal fluids, where it provides the first line of defense against pathogens. ***IgG*** - IgG is a **monomer**, meaning it exists as a single Y-shaped unit, and it does not have a J chain or form polymeric structures. - It is the most abundant antibody in the **blood** and tissue fluids, plays a key role in the secondary immune response, and is the only immunoglobulin that can cross the **placenta**. ***IgM*** - IgM exists as a **pentamer** in its secreted form, consisting of five monomer units linked by a J chain, which is much larger than the dimer shown in the image. - It is the first antibody class produced during a **primary immune response** and is highly effective at activating the complement system. ***IgE*** - IgE is a **monomer** and does not contain a J chain or form polymers. - It is present in very low concentrations in the serum and is primarily involved in **allergic reactions** (type I hypersensitivity) and defense against **parasitic worms**.

Q: Neutrophilic granulomatous mastitis is caused by which of the following organisms?

Corynebacterium kroppenstedtii. ***Corynebacterium kroppenstedtii*** - This organism is the most commonly identified cause of **idiopathic granulomatous mastitis (IGM)**, specifically the subtype characterized by **neutrophilic microabscesses** adjacent to **non-caseating granulomas**. - It is a **lipophilic diphtheroid** often isolated from breast aspirates or biopsies of patients with **chronic, relapsing breast inflammation**. *Corynebacterium minutissimum* - This bacterium is primarily associated with **erythrasma**, a superficial skin infection presenting as reddish-brown patches, especially in intertriginous areas. - It is generally not involved in causing deep-seated, chronic granulomatous inflammation or abscess formation within the breast parenchyma. *Mycobacterium tuberculosis* - *M. tuberculosis* causes **tuberculous mastitis**, which is histologically defined by the presence of **caseating granulomas** and large areas of necrosis. - This differs from neutrophilic granulomatous mastitis, which features predominantly **non-caseating granulomas** mixed with neutrophils and microabscesses. *Staphylococcus aureus* - *S. aureus* is the leading cause of **acute mastitis** and **breast abscesses**, characterized by rapid onset, pain, fever, and abundant acute inflammatory cells (pus). - It is typically associated with acute suppurative changes, not the chronic, indolent, granulomatous tissue reaction found in neutrophilic granulomatous mastitis.

Q: Two different specimens from two patients are received. Which of the following techniques can confirm they are from different individuals (in the context of Mycobacterium tuberculosis)?

RFLP. ***Correct: RFLP*** - **RFLP** (Restriction Fragment Length Polymorphism), particularly using the **IS6110 insertion sequence**, is the traditional gold standard tool for **epidemiological strain typing** of *M. tuberculosis*. - Confirmation that two specimens are from different individuals relies on visualizing two distinct banding patterns, indicating different arrangements of the **IS6110** elements in their respective genomes. - RFLP has **high discriminatory power** for differentiating between strains and establishing epidemiological links. *Incorrect: Mutation analysis* - This technique primarily focuses on detecting **point mutations** in specific genes (e.g., *rpoB*, *katG*) to determine drug susceptibility or resistance patterns. - While differences in resistance profiles suggest different strains, it lacks the **high discriminatory power** of RFLP or VNTR for broad epidemiological linkage assessment. *Incorrect: Pyrosequencing* - Pyrosequencing is a rapid method of DNA sequencing often used to detect **single nucleotide polymorphisms (SNPs)** or specific mutations, such as those related to drug resistance. - It is utilized for sequencing short regions and is not the primary technique for comparing the **large-scale genomic structures** necessary for definitive strain fingerprinting. *Incorrect: RAPD* - **RAPD** (Random Amplification of Polymorphic DNA) is an older PCR-based method that uses arbitrary primers to generate a pattern of amplified DNA fragments. - While used for strain comparison in some bacteria, RAPD is generally considered **less reproducible** and has **lower discriminatory power** than RFLP or VNTR for *M. tuberculosis* typing.

Question 1

Identify the antibody shown in the diagram which has a joining chain and is mainly found in mucosal areas.

Accepted Answer

IgA

Answer

IgM

Answer

IgG

Answer

IgE

Question 2

Neutrophilic granulomatous mastitis is caused by which of the following organisms?

Accepted Answer

Corynebacterium kroppenstedtii

Answer

Corynebacterium minutissimum

Answer

Staphylococcus aureus

Answer

Mycobacterium tuberculosis

Question 3

Two different specimens from two patients are received. Which of the following techniques can confirm they are from different individuals (in the context of Mycobacterium tuberculosis)?

Accepted Answer

RFLP

Answer

Pyrosequencing

Answer

Mutation analysis

Answer

RAPD

INI-CET 2025 — Microbiology