Practice question

Na+ (B) is the right choice. This sits at the heart of Cardiovascular System: Heart: the wording points straight to what is happening in the tissue or organ system. A useful study habit is to ask what the structure or process is for: the functional job described in the stem is exactly what Na+ does in Cardiovascular System: Heart. That reasoning supports Na+. One solid sentence about why this works is enough for recall — you do not need a long essay for every MCQ. When you practise, cover the choices first, write Na+ from memory, then reveal the letter — that habit builds real recall for Cardiovascular System: Heart.