UPSC-CMS 2023 — Microbiology
4 Previous Year Questions with Answers & Explanations
The causative organism of Chancroid is
Which one of the following is the correct sequence of appearance for the Hepatitis B virus serological markers?
Which one of the following statements regarding Gas Gangrene Infection is correct?
Gas Gangrene resulting in crepitus in tissues and a sweet smelling brown exudate is caused due to infection by :
UPSC-CMS 2023 - Microbiology UPSC-CMS Practice Questions and MCQs
Question 1: The causative organism of Chancroid is
- A. Chlamydia trachomatis
- B. Klebsiella granulomatis
- C. Haemophilus ducreyi (Correct Answer)
- D. Psittacosis lymphogranuloma group
Explanation: ***Haemophilus ducreyi*** - **Chancroid** is a sexually transmitted infection characterized by painful genital ulcers and regional lymphadenopathy, caused by the bacterium **Haemophilus ducreyi**. - This bacterium is a **Gram-negative coccobacillus** that requires specific culture media for isolation. *Chlamydia trachomatis* - **Chlamydia trachomatis** is responsible for **chlamydia**, a common STI, and can also cause **lymphogranuloma venereum (LGV)**, which presents with lymphadenopathy and sometimes genital ulcers but not typically the soft, painful ulcers seen in chancroid. - Infections with *Chlamydia trachomatis* are often **asymptomatic** or cause symptoms like urethritis, cervicitis, or proctitis. *Klebsiella granulomatis* - **Klebsiella granulomatis** (formerly *Calymmatobacterium granulomatis*) is the causative agent of **granuloma inguinale** (also known as donovanosis). - **Granuloma inguinale** presents with painless, beefy red, friable ulcerative lesions, which clinically differ from the painful ulcers of chancroid. *Psittacosis lymphogranuloma group* - This option incorrectly groups **psittacosis** (caused by *Chlamydia psittaci*) with **lymphogranuloma venereum (LGV)**, which is caused by specific serovars of *Chlamydia trachomatis*. - **Psittacosis** is primarily a pulmonary infection transmitted by birds, while **LGV** is a sexually transmitted infection distinct from chancroid.
Question 2: Which one of the following is the correct sequence of appearance for the Hepatitis B virus serological markers?
- A. Anti-HBe, HBsAg, Anti-HBc
- B. HBeAg, Anti-HBe, Anti-HBc, HBsAg
- C. HBsAg, HBeAg, Anti-HBc, Anti-HBe (Correct Answer)
- D. Anti-HBc, HBsAg, Anti-HBe
Explanation: ***HBsAg, HBeAg, Anti-HBc, Anti-HBe*** - **HBsAg** (Hepatitis B surface antigen) is the first marker to appear in acute infection, indicating active viral replication. - **HBeAg** (Hepatitis B e-antigen) appears shortly after HBsAg, correlating with high viral replication and infectivity. **Anti-HBc** (antibody to hepatitis B core antigen) is the next to appear, often during the window period. **Anti-HBe** (antibody to hepatitis B e-antigen) signals reduced viral replication and decreased infectivity, typically following the disappearance of HBeAg. *Anti-HBe, HBsAg, Anti-HBc* - This sequence is incorrect because **Anti-HBe** appears much later in the infection, typically after clearance of HBeAg, indicating reduced viral replication. - **HBsAg** is the earliest indicator of active infection, not appearing after Anti-HBe. *HBeAg, Anti-HBe, Anti-HBc, HBsAg* - This sequence is incorrect as **HBeAg** and **Anti-HBe** do not typically appear before **HBsAg**, which is the initial marker of viral presence. - The appearance of **Anti-HBe** before **HBsAg** is also not consistent with the natural history of Hepatitis B infection. *Anti-HBc, HBsAg, Anti-HBe* - This sequence is incorrect because **Anti-HBc** usually appears earlier than Anti-HBe, and while it can be detected relatively early, **HBsAg** is the first antigen to be detectable. - The appearance of **Anti-HBe** is a sign of decreasing viral activity and generally appears later than both HBsAg and Anti-HBc.
Question 3: Which one of the following statements regarding Gas Gangrene Infection is correct?
- A. It is caused by C. perfringens, a gram negative aerobic non-spore-forming bacilli.
- B. It is caused by C. tetani, a gram negative anaerobic non-spore-forming bacilli.
- C. It is caused by C. tetani, a gram positive anaerobic spore-forming bacilli.
- D. It is caused by C. perfringens, a gram positive anaerobic spore-forming bacilli. (Correct Answer)
Explanation: ***It is caused by C. perfringens, a gram positive anaerobic spore-forming bacilli.*** - **Gas gangrene** is primarily caused by **Clostridium perfringens**, which is a **gram-positive**, **anaerobic**, and **spore-forming bacillus**. - These characteristics allow it to thrive in low-oxygen environments characteristic of deep wounds and produce toxins that cause tissue necrosis and gas formation. *It is caused by C. perfringens, a gram negative aerobic non-spore-forming bacilli.* - This statement is incorrect because **C. perfringens** is a **gram-positive** bacterium, not gram-negative. - Furthermore, it is an **anaerobic** organism, meaning it grows in the absence of oxygen, not aerobic. *It is caused by C. tetani, a gram negative anaerobic non-spore-forming bacilli.* - This statement is incorrect because **gas gangrene** is caused by **C. perfringens**, not C. tetani (which causes tetanus). - Also, **C. tetani** is a **gram-positive** bacterium, not gram-negative, and it is **spore-forming**. *It is caused by C. tetani, a gram positive anaerobic spore-forming bacilli.* - This statement is incorrect because, as mentioned, **gas gangrene** is caused by **C. perfringens**, not **C. tetani**. - While **C. tetani** is indeed **gram-positive**, **anaerobic**, and **spore-forming**, it is the causative agent of **tetanus**, not gas gangrene.
Question 4: Gas Gangrene resulting in crepitus in tissues and a sweet smelling brown exudate is caused due to infection by :
- A. Synergistic bacteria
- B. Anaerobic bacteroides spp.
- C. Clostridium perfringens (Correct Answer)
- D. Gas-forming Klebsiella spp.
Explanation: ***Clostridium perfringens*** - **Gas gangrene** is caused by obligate anaerobic, gram-positive rods, notably **Clostridium perfringens**. - This bacterium produces **alpha-toxin (lecithinase)**, which destroys tissues and leads to gas formation (crepitus) and a characteristic **sweet-smelling brown exudate**. *Synergistic bacteria* - While synergistic infections can occur and lead to severe tissue damage, they are not the primary or specific cause of **gas gangrene** with its distinct clinical presentation. - The classic features of gas production and a specific exudate are directly tied to the metabolic activity and toxins of **Clostridial species**. *Anaerobic bacteroides spp.* - **Bacteroides** are common anaerobic bacteria, but they are typically associated with **intra-abdominal abscesses** and **wound infections**, not the specific clinical syndrome of gas gangrene. - They do not produce the potent toxins that lead to extensive gas formation and rapid tissue necrosis characteristic of **Clostridium perfringens**. *Gas-forming Klebsiella spp.* - **Klebsiella spp.** are gram-negative, facultative anaerobic bacteria that can produce gas, especially in deep-seated infections like **emphysematous pyelonephritis** or **liver abscesses**. - However, they do not cause **gas gangrene** with its rapid tissue destruction, crepitus, and sweet-smelling brown exudate, which is pathognomonic for **Clostridial infection**.