Antimicrobial Resistance Practice Questions

Q: Which drug is used to treat Nalidixic acid resistant Salmonella typhi (NARST)?

Azithromycin. **Explanation:** The treatment landscape for Enteric Fever (Typhoid) has evolved significantly due to the emergence of multidrug-resistant (MDR) strains. **1. Why Azithromycin is the Correct Choice:** Nalidixic acid resistance in *Salmonella typhi* is a surrogate marker for "decreased ciprofloxacin susceptibility." When a strain is identified as **NARST (Nalidixic Acid Resistant Salmonella typhi)**, it indicates that fluoroquinolones like Ciprofloxacin will likely result in clinical failure or delayed response. According to current clinical guidelines (including WHO and updated Harrison’s), **Azithromycin** is the preferred oral drug for uncomplicated NARST cases due to its excellent intracellular penetration and low rates of resistance. For complicated or severe NARST, intravenous Ceftriaxone is the drug of choice. **2. Analysis of Incorrect Options:** * **Ciprofloxacin:** Previously the gold standard, it is now avoided in NARST because Nalidixic acid resistance implies mutations in the *gyrA* gene, leading to high-level fluoroquinolone resistance. * **Chloramphenicol:** This was the historical drug of choice but fell out of favor due to the emergence of MDR strains (resistance to Chloramphenicol, Ampicillin, and Septran) and risks of bone marrow toxicity. * **Septran (TMP-SMX):** Like Chloramphenicol, widespread plasmid-mediated resistance makes it unreliable for empirical therapy in the current epidemiological climate. **3. Clinical Pearls for NEET-PG:** * **DOC for Uncomplicated Typhoid:** Ceftriaxone (or Azithromycin if NARST). * **DOC for Carriers:** Ciprofloxacin (for 4–6 weeks) is used for fecal carriers (gallbladder colonization), provided the strain is susceptible. * **Screening:** Nalidixic acid disk diffusion (30 µg) is used in labs to screen for fluoroquinolone resistance. * **Most common cause of death in Typhoid:** Intestinal perforation (usually in the 3rd week).

Q: Burkholderia cepacia is intrinsically resistant to which of the following antibiotics?

Polymyxin B. **Explanation:** *Burkholderia cepacia* complex (Bcc) is a group of Gram-negative, non-fermenting bacilli known for high levels of intrinsic resistance to multiple antibiotics. **1. Why Polymyxin B is the correct answer:** The hallmark of *Burkholderia cepacia* is its **intrinsic resistance to Polymyxins (Polymyxin B and Colistin)**. The underlying mechanism involves a modification of the bacterial lipopolysaccharide (LPS). Specifically, *B. cepacia* incorporates **4-amino-4-deoxy-L-arabinose (L-Ara4N)** into its lipid A. This reduces the negative charge of the outer membrane, preventing the positively charged polymyxin molecules from binding to the cell surface. **2. Analysis of Incorrect Options:** * **A & B (Doxycycline and TMP-SMX):** These are actually among the drugs of choice for treating *B. cepacia* infections. TMP-SMX is often considered the primary treatment option. * **D (Meropenem):** While many Gram-negative bacteria develop resistance to carbapenems, *B. cepacia* is generally susceptible to Meropenem, making it a viable clinical alternative. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Association:** *B. cepacia* is a significant pathogen in patients with **Cystic Fibrosis** and **Chronic Granulomatous Disease (CGD)**. * **Cepacia Syndrome:** A rapid, fatal necrotizing pneumonia and septicemia seen in Cystic Fibrosis patients. * **Laboratory Diagnosis:** It grows on **BCSA (Burkholderia cepacia selective agar)** or **OFPBL medium**, often producing a characteristic "dirt-like" or earthy odor. * **Oxidase Test:** Most strains are weakly oxidase-positive. * **Mnemonic:** Remember **"B.C. is resistant to Colistin"** (B. cepacia). Other Colistin-resistant organisms include *Proteus*, *Providencia*, *Serratia*, and *Burkholderia pseudomallei*.

Q: In the recent decade, *Neisseria gonorrhoeae* has become resistant to all the following drugs, EXCEPT:

Azithromycin. **Explanation:** The correct answer is **Azithromycin**. While resistance to Azithromycin is emerging globally, it remains a component of recommended therapy in many guidelines (often in combination with Ceftriaxone) and has not yet reached the level of "widespread established resistance" seen with the other listed drugs. **1. Why Azithromycin is the correct answer:** Historically, *Neisseria gonorrhoeae* was treated with monotherapy. However, due to rising MICs (Minimum Inhibitory Concentrations), the medical community shifted to **Dual Therapy** (Ceftriaxone + Azithromycin). While "High-level Azithromycin-resistant" (HL-AziR) strains exist, Azithromycin is the only drug among the options that is still clinically relevant in current treatment algorithms for uncomplicated gonorrhea. **2. Analysis of Incorrect Options:** * **Penicillin G:** Resistance emerged in the 1970s via **PPNG** (Penicillinase-producing *N. gonorrhoeae*) strains carrying plasmids. It is no longer used. * **Tetracycline:** Resistance is widespread due to **TRNG** (Tetracycline-resistant *N. gonorrhoeae*) strains, mediated by the *tetM* gene on a transposon. * **Ciprofloxacin:** Fluoroquinolone resistance (due to mutations in *gyrA* and *parC* genes) became so prevalent by the mid-2000s that the CDC removed them from recommended treatment schedules. **3. High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Ceftriaxone (Ceftriaxone 500mg IM single dose is the current CDC recommendation). * **Mechanism of Resistance:** *N. gonorrhoeae* is a "superbug" that utilizes transformation to pick up resistance genes from the environment. * **Culture Media:** Thayer-Martin Medium (Selective medium containing Vancomycin, Colistin, Nystatin, and Trimethoprim). * **Key Feature:** It is a Gram-negative diplococcus (kidney-shaped) found within polymorphonuclear leukocytes (intracellular).

Q: Eth-A gene mutation can lead to cross resistance of Ethionamide with which of the following drugs?

Thioacetazone. **Explanation:** The correct answer is **Thioacetazone**. **Mechanism of Resistance:** Ethionamide is a prodrug used in the treatment of multidrug-resistant tuberculosis (MDR-TB). It requires activation by the monooxygenase enzyme encoded by the **ethA gene**. Mutations in the *ethA* gene lead to a loss of enzyme activity, preventing the conversion of the prodrug into its active form. **Thioacetazone**, another second-line antitubercular drug, is also a prodrug that requires activation by the same **ethA-encoded monooxygenase**. Therefore, a single mutation in the *ethA* gene results in cross-resistance between Ethionamide and Thioacetazone. **Analysis of Incorrect Options:** * **A. PAS (Para-aminosalicylic acid):** Resistance is primarily due to mutations in the *folP* gene or *thyA* gene, affecting the folate synthesis pathway. * **B. Isoniazid (INH):** While Ethionamide and INH share a common structural target (InhA enzyme), leading to cross-resistance via ***inhA* promoter mutations**, the *ethA* mutation specifically affects the activation step unique to Ethionamide and Thioacetazone, not INH (which is activated by *katG*). * **C. Ethambutol:** Resistance is typically due to mutations in the *embB* gene, which encodes arabinosyltransferase. **High-Yield Clinical Pearls for NEET-PG:** * **EthA mutation:** Causes cross-resistance between Ethionamide and Thioacetazone. * **InhA mutation:** Causes cross-resistance between **Isoniazid and Ethionamide** (due to a shared target). * **KatG mutation:** Most common cause of high-level Isoniazid resistance; does **not** cause cross-resistance with Ethionamide. * **Thioacetazone Caution:** It is notorious for causing severe cutaneous adverse drug reactions (e.g., Stevens-Johnson Syndrome), especially in HIV-positive patients.

Question 1

What is the most important mechanism by which Gram-negative bacilli acquire chloramphenicol resistance?

Accepted Answer

Acquisition of a plasmid encoding chloramphenicol acetyltransferase

Answer

Decreased permeability into the bacterial cell

Answer

Lowered affinity of the bacterial ribosome for chloramphenicol

Answer

Switching over from ribosomal to mitochondrial protein synthesis

Question 2

Which drug is used to treat Nalidixic acid resistant Salmonella typhi (NARST)?

Accepted Answer

Azithromycin

Answer

Chloramphenicol

Answer

Ciprofloxacin

Answer

Septran

Question 3

Burkholderia cepacia is intrinsically resistant to which of the following antibiotics?

Accepted Answer

Polymyxin B

Answer

Doxycycline

Answer

TMP-SMX

Answer

Meropenem

Question 4

In the recent decade, *Neisseria gonorrhoeae* has become resistant to all the following drugs, EXCEPT:

Accepted Answer

Azithromycin

Answer

Penicillin G

Answer

Tetracycline

Answer

Ciprofloxacin

Question 5

Eth-A gene mutation can lead to cross resistance of Ethionamide with which of the following drugs?

Accepted Answer

Thioacetazone

Answer

PAS

Answer

Isoniazid

Answer

Ethambutol

Question 6

Which of the following antimicrobial agents is effective against an organism producing extended-spectrum beta-lactamase (ESBL)?

Accepted Answer

Piperacillin-Tazobactam

Answer

Amoxicillin-Clavulanic acid

Answer

Cefepime

Answer

Ceftriaxone

Question 7

A person with prolonged usage of contact lenses presented with irritation of the left eye. After examination, a diagnosis of keratitis was made, and corneal scrapings revealed the growth of Pseudomonas aeruginosa. The bacteria were observed to be multidrug resistant. Which of the following best explains the mechanism of antimicrobial resistance in these isolated Pseudomonas aeruginosa strains?

Accepted Answer

Ability of Pseudomonas to produce biofilms

Answer

Ability to transfer resistance genes from adjacent commensal flora

Answer

Improper contact lens hygiene

Answer

Frequent and injudicious use of topical antibiotics

Question 8

All of the following statements are true regarding Staphylococci except:

Accepted Answer

Expression of methicillin resistance in Staphylococcus aureus increases when it is incubated at 37

C on blood agar

Answer

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

Answer

Beta-lactamase production in Staphylococci is under plasmid control

Answer

Methicillin resistance in Staphylococcus aureus is independent of beta-lactamase production

Question 9

Ureaplasma is naturally resistant to which of the following antibiotics?

Accepted Answer

Cephalosporins

Answer

Erythromycin

Answer

Tetracycline

Answer

Chloramphenicol

Question 10

Drug resistance in Mycobacterium tuberculosis is due to:

Accepted Answer

Mutation

Answer

Conjugation

Answer

Transduction

Answer

None of the above

Antimicrobial Resistance — MCQs

Antimicrobial Resistance — MCQs

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