Which of the following is the primary cause of low serum bicarbonate in a patient with a high anion gap metabolic acidosis?

A.) Excess secretion of bicarbonate from the gastrointestinal tract
B.) Excess secretion of bicarbonate in the urine
C.) Buffering of excess hydrogen ions
D.) Decreased reabsorption from the glomerular proximal tubule
E.) Excessive administration of 0.9% saline