The λ ZAP system from Stratagene/Agilent is most appropriately categorized as which type of vector in its original packaged form?

Introduction / Context:
λ ZAP is a specialized lambda vector system enabling rapid conversion of a recombinant phage clone into a plasmid (e.g., pBluescript) within the host using helper phage–mediated in vivo excision. Recognizing its starting category and special capability avoids classification errors.

Given Data / Assumptions:

• Initial library construction and screening occur in a lambda phage context.
• In vivo excision generates a circular plasmid containing the insert for easy propagation.
• The system is not a cosmid or pure phagemid at the outset.

Concept / Approach:
By embedding a plasmid sequence inside a lambda vector, λ ZAP combines phage-based library screening with plasmid-level convenience. After identifying a positive plaque, excision yields a high-copy plasmid suited for restriction mapping, sequencing, and subcloning without re-cloning steps.

Step-by-Step Solution:

Verification / Alternative check:
Vendor manuals describe λ ZAP as a lambda vector with excisable plasmid arms, not as a cosmid or basic phagemid.

Why Other Options Are Wrong:

• B: While excision yields a plasmid related to phagemid features, the starting vector is lambda phage.
• C: Insert capacities and architecture differ from cosmids.
• D/E: Do not describe the lambda-based excision system.

Common Pitfalls:
Assuming that because the end product is a plasmid, the starting vector must be a plasmid or phagemid; λ ZAP starts as lambda.

Final Answer:
Lambda phage vector that allows in vivo excision to a plasmid