Antimicrobial pharmacology questions.

Answer: Blocking transpeptidase cross-linking of the wall

Beta-lactams bind penicillin-binding proteins, the transpeptidases that cross-link peptidoglycan strands. Without cross-linking the wall cannot withstand osmotic pressure and the cell lyses. This also explains why they are bactericidal and why they do nothing to organisms lacking that wall.

Answer: Binds the D-Ala-D-Ala terminus, not the enzyme

Vancomycin attacks the same wall-building process from the other side: instead of inhibiting the enzyme, it caps the substrate the enzyme needs. That is why beta-lactamase does not affect it, and its size is why it cannot cross the Gram-negative outer membrane.

Answer: Direct histamine release driven by infusion rate

The reaction is pharmacological, not immunological: vancomycin causes mast cells to degranulate directly, without any antibody involved. Because the trigger is rate rather than exposure, slowing the infusion usually solves it, and the patient is not truly allergic.

Answer: Nephrotoxicity together with ototoxicity

Aminoglycosides accumulate in the renal proximal tubule and in the hair cells of the inner ear, and both tissues are slow to recover. Because killing is concentration-dependent while toxicity tracks trough levels, once-daily dosing gives high peaks with a low trough and improves both.

Answer: Inhibits CYP3A4 from the very first dose given

Macrolides are potent CYP3A4 inhibitors, and inhibition is immediate because it acts on enzyme already present. That is the opposite of induction, which needs new enzyme to be made and therefore takes days to appear and days to wear off after stopping.

Answer: Divalent cations chelate it and block absorption

Quinolones bind calcium, magnesium, iron and aluminium in the gut to form a complex too large to be absorbed. Nothing is wrong with the drug or the milk individually; the problem is that they meet. Separating doses by a couple of hours resolves it, and tetracyclines share the trap.

Answer: Acetaldehyde accumulates, giving a disulfiram reaction

Metronidazole inhibits aldehyde dehydrogenase, the enzyme clearing acetaldehyde produced when ethanol is broken down. Acetaldehyde accumulates and causes the flushing, nausea and palpitations that make the reaction memorable, which is why disulfiram was used deliberately for the same effect.

Answer: Bind calcium in developing teeth and bone

The chelation that ruins tetracycline absorption with milk is the same chemistry that causes the problem in children: the drug binds calcium wherever it finds it, including in teeth and growing bone, producing permanent discolouration. One mechanism explains interaction and contraindication.

Answer: The spelling records the genus that produced each

The single letter records which genus the compound was originally isolated from: -micin from Micromonospora, -mycin from Streptomyces. It is a historical label rather than a pharmacological one, which is exactly why it is worth knowing — it looks like it should mean something clinically.

Answer: Trimethoprim combined with sulfamethoxazole

The two drugs block consecutive steps of the same folate pathway, sulfamethoxazole earlier and trimethoprim later, which is why the pair is synergistic rather than merely additive. The other combinations pair an antibiotic with a beta-lactamase inhibitor, protecting the drug rather than attacking a second step.