Purebred dogs are prone to genetically determined abnormalities.
Conclusion
Potential dog owners who want to reduce the risk of incurring costly medical bills for their pets would be well advised to choose nonpurebred dogs.
Evidence
Purebred dogs are prone to genetically determined abnormalities.
Such abnormalities often can be corrected by surgery, which can cost several thousand dollars.
Nonpurebred dogs rarely suffer from these genetically determined abnormalities.
Evaluate
The argument here ties the decision of which dog to choose directly to the financial impact of genetically determined abnormalities. Since purebred dogs are more prone to these abnormalities and correcting them surgically can be expensive, the author concludes that choosing nonpurebred dogs would be financially wiser.
The argument implies that the financial burden of these surgeries is significant enough that it's the primary or sole consideration for choosing nonpurebred dogs over purebred dogs. To weaken this argument, we need to show either that genetically determined abnormalities are not as significant of a factor in determining medical costs, or that other factors might offset or overshadow this cost consideration.
Goal
Let's find an answer that diminishes the impact of genetically determined abnormalities on financial decision-making for potential dog owners.