This patient looks like someone with somatic mosaicism. Mosaic individuals acquired their ADPKD not at conception (therefore negative family history) but during early development when only one polypotent stem cell in a multicell embryo acquired a de novo mutation. So their disease is diluted and presents focally or with extremely asymmetry as in your patient.
Conventional mutation screen is typically negative is the signal from the mutant alleles is greatly diluted. We have developed a Next Generation Sequencing protocol for this type of mutations and can detect them down to 5% level. If you are interested, we can test this patient.
The plot thickens. Our radiologist reports the following on this scan:
Large complex / multiloculated cystic left renal mass as described above. The differential diagnosis includes cystic renal cell carcinoma and multilocular cystic nephroma. The appearance of this mass is incompatible with focal polycystic renal disease. Urologic consultation is recommended.
Based on this, urology is removing the kidney. We will see what the pathology shows...