NEET-PG 2012 — Microbiology

Q: Exanthema subitum is caused by -

HHV. ***HHV*** - **Exanthema subitum**, also known as **roseola infantum**, is most commonly caused by **Human Herpesvirus 6 (HHV-6)** and sometimes **Human Herpesvirus 7 (HHV-7)**. - It is characterized by high fever followed by a **maculopapular rash** once the fever subsides. *HIV* - **Human Immunodeficiency Virus (HIV)** causes **Acquired Immunodeficiency Syndrome (AIDS)**, leading to a weakened immune system. - While HIV can cause various skin manifestations, **exanthema subitum** is not one of its primary or characteristic presentations. *HCV* - **Hepatitis C Virus (HCV)** primarily causes **hepatitis**, infecting the liver and potentially leading to chronic liver disease. - HCV is not known to cause **exanthema subitum** or similar childhood exanthems. *HPV* - **Human Papillomavirus (HPV)** is responsible for **warts** and certain **cancers**, particularly cervical cancer. - HPV does not cause **exanthema subitum**; its manifestations are typically localized to skin and mucous membranes as proliferative lesions.

Q: A child with fever with abdominal cramps & pus in stools, causative organism is ?

EAEC. ***EAEC (Enteroaggregative E. coli)*** - Among the E. coli strains listed, EAEC is the answer for this question, though **this is an atypical presentation**. - EAEC classically causes **persistent watery diarrhea** (>14 days) in children, often with **low-grade fever** and **abdominal pain**. - While EAEC primarily causes non-bloody diarrhea, it can occasionally produce **mucoid stools** with inflammatory cells due to mucosal inflammation from biofilm formation. - **Note:** The classic organism for fever + cramps + pus in stools would be **Shigella**, **Campylobacter**, or **EIEC (Enteroinvasive E. coli)** - not listed here. *EHEC (Enterohemorrhagic E. coli)* - EHEC (O157:H7) causes **hemorrhagic colitis** with bloody diarrhea due to **Shiga toxins**. - Can lead to **hemolytic uremic syndrome (HUS)** in children. - Characterized by **blood** rather than pus in stools, distinguishing it from typical dysentery. *ETEC (Enterotoxigenic E. coli)* - Most common cause of **traveler's diarrhea** and watery diarrhea in developing countries. - Produces **heat-labile (LT)** and **heat-stable (ST)** enterotoxins causing secretory diarrhea. - Results in profuse **watery stools without inflammation, blood, or pus**. *EPEC (Enteropathogenic E. coli)* - Leading cause of infantile diarrhea in developing countries. - Causes **attaching and effacing** lesions on intestinal mucosa. - Results in **watery diarrhea without significant inflammatory cells or pus** in stools.

Q: Classical complement is activated by:

Ag-Ab complex. ***Ag-Ab complex*** - The **classical complement pathway** is initiated by the binding of **C1q** to an antigen-antibody complex, specifically involving **IgM** or certain subclasses of **IgG**. - This binding triggers a cascade of events leading to the activation of the complement system, ultimately resulting in the **lysis of target cells**, **opsonization**, and **inflammation**. *C3 Convertase* - **C3 convertase** is an enzyme complex formed later in the complement cascade, responsible for cleaving C3 into C3a and C3b. - While essential for all complement pathways, it is a **downstream effector** and not the initial activator of the classical pathway. *C1* - **C1** is a complex protein that includes C1q, C1r, and C1s. While C1 plays a crucial role in the classical pathway, it is **activated by** the antigen-antibody complex, not an independent activator. - The activation sequence is: **Ag-Ab complex → C1q binding → C1 activation → cascade initiation**. Thus, the Ag-Ab complex is the primary trigger, and C1 is the responder. *IgA* - **IgA** primarily functions in mucosal immunity and is generally **not an activator** of the classical complement pathway. - Instead, IgA can activate the **alternative complement pathway** under specific circumstances, but not the classical pathway through direct binding to C1q.

83 Questions — Page 9 of 9

Q81

Exanthema subitum is caused by -

Q82

A child with fever with abdominal cramps & pus in stools, causative organism is ?

Q83

Classical complement is activated by: