ICAR AICE-JRF/SRF Veterinary Chemotherapy & Antimicrobial Stewardship: practice questions

Question 1 · medium

Methicillin resistance in Staphylococcus pseudintermedius is due to

1. loss of outer-membrane porins that admit beta-lactams to the cell
2. erm methylation of 23S rRNA in the large ribosomal subunit
3. mecA encoding PBP2a, a low-affinity penicillin-binding protein
4. blaZ encoding a penicillinase that clavulanate cannot inhibit

Question 2 · hard

An E. coli isolate is resistant to ceftiofur and cefoxitin, and the resistance is not reversed by clavulanic acid. Which enzyme type is most likely?

1. TEM-1 narrow-spectrum penicillinase
2. CTX-M extended-spectrum beta-lactamase
3. Plasmid-mediated AmpC such as CMY-2
4. Staphylococcal blaZ penicillinase

Question 3 · hard

Which pair correctly names the two main acquired tetracycline-resistance mechanisms with an example gene for each?

1. Enzymatic acetylation by cat; target methylation by erm
2. Efflux by tet(M); ribosomal protection by tet(A)
3. Efflux by tet(A); ribosomal protection by tet(M)
4. Porin loss by ompF; target mutation in the gyrA gene

Question 4 · hard

Macrolides reach higher concentrations in milk than in plasma after systemic dosing of a cow mainly because

1. mammary epithelium secretes them actively by P-glycoprotein
2. they bind milk fat globules covalently and stay in the udder
3. they are weak acids ion-trapped in slightly alkaline milk
4. they are weak bases ion-trapped in slightly acidic milk

Question 5 · hard

An E. coli isolate's ciprofloxacin MIC is raised slightly; sequencing finds no gyrA or parC mutation but a plasmid gene. Which gene best explains this?

1. mcr-1, encoding a lipid A phosphoethanolamine transferase
2. mecA, encoding a low-affinity penicillin-binding protein
3. erm(B), encoding a 23S rRNA methyltransferase enzyme
4. qnr, encoding a protein that protects DNA gyrase

Question 6 · hard

The colistin-resistance gene mcr-1, first reported in 2015 from pigs and people in China, is especially worrying because it

1. is plasmid-borne and so can spread horizontally between bacteria
2. is a chromosomal mutation that cannot leave its host bacterium
3. can only be expressed in Gram-positive bacteria such as enterococci
4. also inactivates every beta-lactam including the carbapenems

Question 7 · hard

The 'mutant selection window' for an antimicrobial is the concentration range

1. between the Cmax and the trough within one dosing interval
2. below the MIC, where no bacteria are inhibited
3. above the MPC, where all single-step mutants die
4. between the MIC and the mutant prevention concentration

Question 8 · medium

A beta-lactam is time-dependent. Which change in regimen most improves its efficacy against a susceptible organism?

1. Doubling the single dose but keeping the interval
2. Giving the whole daily dose as one bolus
3. Adding a bacteriostatic tetracycline to it
4. Giving the same daily dose more frequently

Question 9 · medium

After an IV bolus a drug reaches 16 µg/mL and has a half-life of 2 h. For an MIC of 1 µg/mL and a 12-hour dosing interval, roughly what percentage of the interval is spent above the MIC?

1. 50%
2. 100%
3. 33%
4. 67%

Question 10 · hard

An E. coli isolate from a dog's urine has an MIC above the epidemiological cut-off (ECOFF) but below the clinical susceptible breakpoint for that drug at the urinary site. What does this mean?

1. It likely has an acquired mechanism yet is still predicted to respond
2. It must be reported as resistant because the ECOFF is exceeded
3. It is wild-type and clinically resistant at the urinary site
4. It is fully wild-type, as ECOFFs and breakpoints are identical