What it is a part of

нога foot; leg
пятка левой ногиg the heel of the left foot
NB Note that neither ступниg "foot" nor подошвыg "foot" can be substituted for ногиg here. Note also that in some contexts, mostly in formal style, the word пятки (plural) can combine with the plural form ногg:
При рождении оказалось, что у ребёнка деформированы пятки ног. When the child was born it turned out that its heels were deformed.