Sentence:  Serum <WSD>potassium</WSD> at this time was 3.8 mEq. per liter , and the hemoglobin was 13.9 gm. By Dec. 1 , 1958 , the weakness in the pelvic and quadriceps muscle groups was appreciably worse , and it became difficult for the patient to rise unaided from a sitting or reclining position . 
Meaning of the word marked with <WSD> tags:  a light soft silver-white metallic element of the alkali metal group; oxidizes rapidly in air and reacts violently with water; is abundant in nature in combined forms occurring in sea water and in carnallite and kainite and sylvite