Antimicrobial pharmacology
Practice questions with explanations

C. Chelation with calcium, magnesium, aluminium, iron and zinc in antacids, supplements and dairy products

The resulting complexes are not absorbed and the loss of bioavailability can be enough to cause treatment failure. Separating doses by at least two hours before or four hours after the cation resolves it. This is a common and easily missed interaction because supplements and antacids are frequently not reported as medicines.

B. Aminoglycosides

Aminoglycoside killing depends on the ratio of peak concentration to MIC, and they show a prolonged post-antibiotic effect, so one large daily dose kills more effectively than divided doses while allowing a trough low enough to limit renal and cochlear uptake. Beta-lactams are time-dependent: their efficacy tracks the time the concentration stays above MIC, which is why they are given frequently or by extended infusion.

D. It hydrolyses penicillins and most cephalosporins, so treatment usually relies on a carbapenem or another agent guided by susceptibilities

ESBLs are plasmid-encoded enzymes in Gram-negatives, chiefly Escherichia coli and Klebsiella, that destroy penicillins and third-generation cephalosporins, and the plasmids often carry resistance to other classes as well. Carbapenems are the usual reliable option, with nitrofurantoin or fosfomycin sometimes available for lower urinary tract infection. Their spread is a major driver of carbapenem use and therefore of carbapenem resistance.