A

Reasoning: For arterial blood gas sampling the syringe is only coated with a thin film of dry (or liquid-then-expelled) heparin; keeping 0.3 mL of liquid heparin inside the syringe would markedly dilute the 1 mL blood sample and alter pH, PCO₂ and electrolytes, so this practice is specifically discouraged [Ordog 1985, PMID 3919622; Higgins 2007, PMID 18018679]. Normal pH/HCO₃⁻/PCO₂ values can mask mixed disorders, a negative modified Allen test mandates another puncture site, and the radial artery is recommended because of its accessibility and collateral circulation [Franklin 1995, PMID 10150503].