Which prefix denotes 'under' or 'below'?

The prefix that denotes 'under' or 'below' is 'hypo-'. This prefix is commonly used in medical terminology to indicate conditions that are less than normal or below a certain threshold. For example, in the term 'hypoglycemia', 'hypo-' signifies lower than normal levels of glucose in the blood.

The other prefixes listed do not convey the same meaning. 'Sym-' refers to 'together' or 'with', as seen in words like 'symptom', which indicates a sign that occurs with certain conditions. 'Pachy-' means 'thick', often used in terms related to abnormal thickness, such as 'pachycephaly' (thickened skull). 'Pano-' means 'all' or 'entire', as used in 'panorama', which means an all-encompassing view. Thus, 'hypo-' is the only prefix that specifically indicates an 'under' or 'below' condition.