The blood pressure of and Individual is 140 mm Hg. In this statement ‘Hg’ refers to

The correct answer is B. In blood pressure readings, Hg is the chemical symbol of mercury, from the Latin hydrargyrum (liquid silver). A reading of 140 mm Hg means the pressure is strong enough to push a column of mercury 140 millimetres high in a sphygmomanometer.

• A) Hydrogen is H, not Hg. Its symbol comes from the Greek for water-former, and it has nothing to do with pressure measurement.
• C) Hygrometer is an instrument that measures humidity. Its name does not give a symbol for the unit.
• D) Heliograph is a signalling device that uses sunlight reflected by a mirror. It is unrelated to blood pressure.

The core concept is that pressure units such as mm Hg and the older torr are defined by the height of a mercury column. This is basic physics, and it links to the general science and everyday health-literacy questions that appear in Prelims. In Mains, it can be a small but useful point when discussing diagnostic equipment and the phase-out of mercury devices.

Trap: Some candidates think of Hg only as a chemical symbol and forget its use as a pressure unit. Others pick hydrogen because H appears in the symbol. Remember that mm Hg is a unit of pressure derived from mercury.