Antimicrobial Resistance Practice Questions

Q: Haemophilus influenzae has been isolated from the CSF of a two-year-old boy suffering from meningitis. The strain is beta-lactamase producing and resistant to chloramphenicol. What is the most appropriate antimicrobial in this situation?

Third-generation cephalosporin. ### Explanation **1. Why Third-generation Cephalosporins are Correct:** The treatment of choice for *Haemophilus influenzae* meningitis, especially in the pediatric age group, is a **third-generation cephalosporin** (e.g., **Ceftriaxone** or **Cefotaxime**). These drugs are highly effective because they possess excellent penetration into the cerebrospinal fluid (CSF) and are stable against the beta-lactamases produced by resistant strains. Since the isolate in this case is both beta-lactamase producing (rendering Ampicillin ineffective) and Chloramphenicol-resistant, third-generation cephalosporins remain the gold standard for empirical and definitive therapy. **2. Why Other Options are Incorrect:** * **Trimethoprim–sulfamethoxazole (A):** While it has some activity against *H. influenzae*, it has poor CSF penetration and high rates of resistance, making it unsuitable for life-threatening meningitis. * **Ciprofloxacin (B):** Fluoroquinolones are generally avoided as first-line agents for meningitis in young children due to potential concerns regarding cartilage toxicity and the availability of safer, more effective alternatives. * **Vancomycin (D):** Vancomycin is primarily active against Gram-positive organisms (like MRSA or resistant *S. pneumoniae*). It has no significant activity against Gram-negative organisms like *H. influenzae*. **3. Clinical Pearls for NEET-PG:** * **Resistance Mechanism:** Beta-lactamase production in *H. influenzae* is usually mediated by the **TEM-1 plasmid**. * **Satellite Phenomenon:** *H. influenzae* grows on blood agar around *S. aureus* colonies because the latter provides **Factor V (NAD)**. * **Drug of Choice:** For *H. influenzae* epiglottitis or meningitis, always prioritize **Ceftriaxone**. * **Prophylaxis:** **Rifampicin** is the drug of choice for chemoprophylaxis in close contacts of a patient with *H. influenzae* type b (Hib) meningitis.

Q: A 60-year-old woman hospitalized for three weeks with widely metastatic ovarian adenocarcinoma becomes septic with vancomycin-resistant enterococcus. What is the mechanism of vancomycin resistance in this organism?

Formation of novel cell wall peptide bridges. **Explanation:** The correct answer is **D. Formation of novel cell wall peptide bridges.** **Mechanism of Resistance:** Vancomycin normally acts by binding to the **D-Ala-D-Ala** terminus of peptidoglycan precursors, preventing the transpeptidation and transglycosylation steps necessary for cell wall synthesis. In Vancomycin-Resistant Enterococci (VRE), the organism acquires genes (most commonly **VanA** or **VanB**) that alter this target. The terminal D-Alanine is replaced with **D-Lactate** (or occasionally D-Serine). This structural change results in the formation of novel peptide bridges (D-Ala-D-Lac) that have a significantly decreased affinity for vancomycin, allowing cell wall synthesis to continue despite the presence of the drug. **Analysis of Incorrect Options:** * **A. Acetylation of antibiotic:** This is a common mechanism for resistance against **Aminoglycosides** (via aminoglycoside-modifying enzymes). * **B. Altered drug-binding protein:** This refers to altered **Penicillin-Binding Proteins (PBPs)**, which is the mechanism for Methicillin-resistant *Staphylococcus aureus* (MRSA) and penicillin-resistant *Streptococcus pneumoniae*. * **C. Beta-lactamase production:** This is the mechanism used by many bacteria (e.g., *S. aureus*, *H. influenzae*) to hydrolyze the beta-lactam ring of penicillins and cephalosporins. **High-Yield NEET-PG Pearls:** * **VRE Risk Factors:** Prolonged hospitalization, ICU stay, and prior use of vancomycin or cephalosporins. * **Genetics:** The *VanA* gene is often carried on a **transposon (Tn1546)**, facilitating its spread. * **Treatment of Choice:** Linezolid, Quinupristin-Dalfopristin, or Tigecycline are typically used for VRE infections. * **D-Ala-D-Lac vs. D-Ala-D-Ser:** D-Ala-D-Lac provides high-level resistance (VanA, VanB), while D-Ala-D-Ser provides low-level resistance (VanC).

Q: Which of the following statements regarding MRSA is true?

Pulsed-field gel electrophoresis is used for MRSA typing.. **Explanation:** **Correct Answer: D. Pulsed-field gel electrophoresis is used for MRSA typing.** Pulsed-field gel electrophoresis (PFGE) is considered the "gold standard" for the molecular typing and epidemiological investigation of MRSA outbreaks. It allows for the separation of large DNA fragments, creating a unique "fingerprint" that helps clinicians track the transmission of specific strains within a hospital or community. **Analysis of Incorrect Options:** * **Option A:** Ceftriaxone is a 3rd generation cephalosporin. MRSA is inherently resistant to almost all beta-lactams (including ceftriaxone). Vancomycin or Linezolid are preferred for systemic infections, while Mupirocin is used for superficial decolonization. * **Option B:** While many *S. aureus* strains produce beta-lactamase (penicillinase), the defining feature of MRSA is not enzyme production, but an **altered target site**. * **Option C:** The *mecA* gene indeed encodes for **PBP2a**; however, PBP2a has a **low affinity** for beta-lactam antibiotics. This low affinity prevents the antibiotic from binding to the bacterial cell wall, allowing peptidoglycan synthesis to continue despite the presence of the drug. **High-Yield Clinical Pearls for NEET-PG:** * **Screening:** Cefoxitin disk diffusion is the preferred method for detecting methicillin resistance in the lab (it is a better inducer of the *mecA* gene than oxacillin). * **Genetic Basis:** Resistance is carried on the **SCCmec** (Staphylococcal Cassette Chromosome mec) element. * **Exceptions:** The only beta-lactams with activity against MRSA are 5th generation cephalosporins (e.g., **Ceftaroline**, Ceftobiprole). * **DOC:** Vancomycin remains the drug of choice for serious MRSA infections.

Q: Which disease is transmitted by Aedes aegypti?

Yellow fever. **Explanation:** The correct answer is **Yellow fever**. *Aedes aegypti* is a highly efficient urban vector known for its "sip-feeding" habit (biting multiple people to complete one meal), which facilitates the rapid spread of viral outbreaks. **1. Why Yellow Fever is Correct:** Yellow fever is caused by a Flavivirus and is primarily transmitted by *Aedes aegypti* in urban cycles. In the jungle cycle, it is transmitted by *Haemagogus* and *Sabethes* mosquitoes. It is a mandatory reportable disease under International Health Regulations (IHR). **2. Why Other Options are Incorrect:** * **Japanese Encephalitis (JE):** This is transmitted by **Culex** mosquitoes, specifically *Culex tritaeniorhynchus*. The virus cycles between pigs (amplifier hosts), water birds, and humans (dead-end hosts) in rice-field ecosystems. * **Kyasanur Forest Disease (KFD):** Also known as "Monkey Fever," this is a tick-borne viral hemorrhagic fever. The primary vector is the **hard tick (*Haemaphysalis spinigera*)**. It is endemic to the Western Ghats of India. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diseases transmitted by *Aedes aegypti*:** Remember the mnemonic **D-C-Y-Z** (Dengue, Chikungunya, Yellow Fever, Zika). * **Vector Characteristics:** *Aedes* mosquitoes are "day-biters," breed in artificial collections of clean water (coolers, flower pots), and are known as "tiger mosquitoes" due to white stripes on their bodies. * **Yellow Fever Vaccine:** The **17D vaccine** is a live-attenuated vaccine. Immunity starts after 10 days and is now considered valid for life for international travel purposes. * **Councilman Bodies:** These are acidophilic apoptotic hepatocytes seen on liver biopsy in Yellow Fever patients.

Question 1

Haemophilus influenzae has been isolated from the CSF of a two-year-old boy suffering from meningitis. The strain is beta-lactamase producing and resistant to chloramphenicol. What is the most appropriate antimicrobial in this situation?

Accepted Answer

Third-generation cephalosporin

Answer

Trimethoprim–sulfamethoxazole combination

Answer

Ciprofloxacin

Answer

Vancomycin

Question 2

A 60-year-old woman hospitalized for three weeks with widely metastatic ovarian adenocarcinoma becomes septic with vancomycin-resistant enterococcus. What is the mechanism of vancomycin resistance in this organism?

Accepted Answer

Formation of novel cell wall peptide bridges

Answer

Acetylation of antibiotic

Answer

Altered drug-binding protein

Answer

Beta-lactamase production

Question 3

Methicillin-resistant staphylococci do not respond to beta-lactam antibiotics because:

Accepted Answer

They have acquired penicillin-binding protein which has low affinity for beta-lactam antibiotics.

Answer

They produce a beta-lactamase which destroys methicillin and related drugs.

Answer

They elaborate an amidase which destroys methicillin and related drugs.

Answer

They are less permeable to beta-lactam antibiotics.

Question 4

All of the following statements are true about Staphylococci except?

Accepted Answer

Expression of methicillin resistance in Staphylococcus aureus increases when it is incubated at 37 degrees Celsius on blood agar.

Answer

A majority of infections caused by coagulase-negative Staphylococci are due to Staphylococcus epidermidis.

Answer

Beta-Lactamase production is under plasmid control.

Answer

Methicillin resistance in Staphylococcus aureus is independent of beta-Lactamase production.

Question 5

Which of the following statements regarding MRSA is true?

Accepted Answer

Pulsed-field gel electrophoresis is used for MRSA typing.

Answer

Ceftriaxone is preferred for the treatment of superficial MRSA infections.

Answer

MRSA isolates produce beta-lactamase.

Answer

The mecA gene of MRSA encodes for penicillin-binding protein 2a (PBP2a), which has a high affinity for beta-lactam antibiotics.

Question 6

Which of the following is an antimicrobial susceptibility testing method?

Accepted Answer

NCCLS

Answer

Kirby-Bauer method

Answer

ATCC

Answer

NCTC

Question 7

Which of the following is NOT a bacterial drug resistance mechanism?

Accepted Answer

Inhibition of DNA gyrase or topoisomerase activity by the bacteria.

Answer

Synthesis of enzymes that inactivate antibiotics.

Answer

Alteration of porin channels to reduce drug entry.

Answer

Active efflux of the drug out of the bacterial cell.

Question 8

What is the most common mechanism for the transfer of resistance in Staphylococcus aureus?

Accepted Answer

Transduction

Answer

Conjugation

Answer

Transformation

Answer

Mutation

Question 9

Which disease is transmitted by Aedes aegypti?

Accepted Answer

Yellow fever

Answer

Japanese encephalitis

Answer

Kyasanur Forest Disease

Answer

All of the above

Question 10

What is the minimum inhibitory concentration (MIC)?

Accepted Answer

Minimum concentration of an antibiotic which prevents visible growth of a bacterium

Answer

The concentration of antibiotic which kills 99.9% of the bacteria

Answer

Maximum concentration of an antibiotic which prevents visible growth of a bacterium

Answer

None of the above

Antimicrobial Resistance — MCQs

Antimicrobial Resistance — MCQs

On this page

Practice by Chapter

Want unlimited practice?