Answer:
The gene that would most likely lower blood pH of a human upon RNAI is NHE3. NHE3 encodes for a protein called sodium-hydrogen exchanger 3, which plays an important role in maintaining acid-base balance in the body. When NHE3 is inhibited, it reduces the excretion of acid in the urine and leads to an accumulation of acid in the blood, resulting in a decrease in blood pH. The other genes mentioned - SGLT, GLUT, Pendrin, and NKCC2 - are not directly involved in acid-base balance and therefore would not be expected to have a significant effect on blood pH upon RNAI.