Final answer:
The most important poor prognostic factor for post-streptococcal glomerulonephritis is low C3 levels.
Step-by-step explanation:
The most important poor prognostic factor for post-streptococcal glomerulonephritis (PSGN) is low C3 levels. PSGN is an immune complex disease that occurs as a result of an inflammatory response triggered by immune complexes formed between Streptococcus pyogenes antigens and antibodies. These immune complexes deposit in the glomeruli of the kidney and cause damage and inflammation. Lower levels of C3 indicate more severe immune complex deposition and are associated with a poorer prognosis.