What can smear in a restriction digest indicate?

Correct answer: Degraded DNA or nonspecific cutting

The direct answer is B: a smear can indicate degraded DNA or nonspecific cutting. In a clean restriction digest, DNA fragments usually appear as distinct bands at particular positions. A smear is a continuous, spread-out signal rather than a sharp band, so it suggests that the DNA population contains many fragment sizes or that the sample was damaged. Step by step, degradation may break DNA randomly into many pieces; alternatively, star activity or another nonspecific process may cut at extra sites and create many sizes. Poor DNA quality, contaminated reagents, excessive enzyme, unsuitable buffer, overlong incubation, or harsh handling may contribute. Option B is correct. Option A is wrong because an exact clean digestion generally gives expected discrete bands, not always a smear. Option C is wrong because intact plasmid commonly gives characteristic conformational bands rather than a digestion smear. Option D is wrong because if no DNA were loaded, there would normally be no DNA signal, not a DNA smear. Check the untreated control, DNA quality, enzyme amount, buffer, temperature, and incubation time before drawing a conclusion.