hi im dr satya
welcome to health care magic.
painful oval lesions on the back of the knee having a brown or deep red in color points towards some erythema
or an acute eczema
. It also depends on whether the lesion has any infection on it or if she is diabetic.
Any ways it is best if she is taken for a consultation with a dermatologist
i hope and wish for a sppedy recovery of your mother.