UPSC-CMS 2025 — Microbiology

Q: Which one of the following is the causative organism of Erythrasma, a mild, localized and superficial skin infection?

Corynebacterium minutissimum. ***Corynebacterium minutissimum*** - This bacterium is the **primary causative agent** of erythrasma, a superficial skin infection. - It produces **porphyrins**, which fluoresce with a characteristic **coral-red color** under a Wood's lamp, aiding in diagnosis. *Corynebacterium matruchotii* - This species is typically found in the **oral cavity** and is associated with dental plaque and periodontal diseases. - It is **not known** to cause erythrasma or superficial skin infections. *Corynebacterium pseudotuberculosis* - This bacterium is a significant pathogen in **animals**, particularly livestock, causing conditions like caseous lymphadenitis. - While it can *rarely* cause infections in humans (e.g., lymphadenitis), it is **not associated** with erythrasma. *Corynebacterium diphtheriae* - This is the causative agent of **diphtheria**, a severe respiratory illness characterized by a pseudomembrane in the throat. - It causes systemic toxemia and is **not responsible** for localized, superficial skin infections like erythrasma. [Practice more Microbiology PYQs on OnCourse]

2 Previous Year Questions with Answers & Explanations

Questions

Which one of the following statements is correct in the diagnosis of Giardiasis?

Which one of the following is the causative organism of Erythrasma, a mild, localized and superficial skin infection?

UPSC-CMS 2025 - Microbiology UPSC-CMS Practice Questions and MCQs

Question 1: Which one of the following statements is correct in the diagnosis of Giardiasis?

A. String test is done to find out cysts of Giardia lamblia
B. Jejunal biopsy samples can show presence of larvae of Giardia lamblia
C. Cystic form of Giardia lamblia remains viable in water upto 1 week only
D. Stool sample at 2-3 days interval should be examined for cysts (Correct Answer)

Explanation: ***Stool sample at 2-3 days interval should be examined for cysts*** - Due to the **intermittent shedding** of *Giardia lamblia* cysts, multiple stool samples collected over several days (e.g., 2-3 days apart for a total of three samples) significantly increase the sensitivity of detection. - Microscopic examination of these samples for the presence of **cysts** is a primary diagnostic method for giardiasis. *String test is done to find out cysts of Giardia lamblia* - The **string test** (or Entero-Test) is primarily used to collect **trophozoites** from the duodenum, not cysts. - Cysts are typically found in **feces**, while trophozoites are free-living in the upper small intestine. *Jejunal biopsy samples can show presence of larvae of Giardia lamblia* - *Giardia lamblia* exists as **trophozoites** and **cysts**, not larvae. - While trophozoites can be seen in jejunal biopsies, they are **protozoan parasites**, not helminthic larvae. *Cystic form of Giardia lamblia remains viable in water upto 1 week only* - *Giardia lamblia* cysts are **highly resistant** and can remain viable in cold water for **months**, much longer than one week. - This extreme viability contributes to the widespread transmission of giardiasis through contaminated water sources.

Question 2: Which one of the following is the causative organism of Erythrasma, a mild, localized and superficial skin infection?

A. Corynebacterium matruchotii
B. Corynebacterium minutissimum (Correct Answer)
C. Corynebacterium pseudotuberculosis
D. Corynebacterium diphtheriae

Explanation: ***Corynebacterium minutissimum*** - This bacterium is the **primary causative agent** of erythrasma, a superficial skin infection. - It produces **porphyrins**, which fluoresce with a characteristic **coral-red color** under a Wood's lamp, aiding in diagnosis. *Corynebacterium matruchotii* - This species is typically found in the **oral cavity** and is associated with dental plaque and periodontal diseases. - It is **not known** to cause erythrasma or superficial skin infections. *Corynebacterium pseudotuberculosis* - This bacterium is a significant pathogen in **animals**, particularly livestock, causing conditions like caseous lymphadenitis. - While it can *rarely* cause infections in humans (e.g., lymphadenitis), it is **not associated** with erythrasma. *Corynebacterium diphtheriae* - This is the causative agent of **diphtheria**, a severe respiratory illness characterized by a pseudomembrane in the throat. - It causes systemic toxemia and is **not responsible** for localized, superficial skin infections like erythrasma.