In “pharming” applications, transgenic goats are engineered to produce a therapeutic variant of human tissue-type plasminogen activator (tPA). In which bodily fluid or tissue is this recombinant protein harvested most efficiently?

Introduction / Context:
Pharming refers to producing high-value therapeutic proteins in the milk, eggs, or other secretions of transgenic animals. One classic example is expressing human tissue-type plasminogen activator (tPA), a thrombolytic drug, in the mammary gland of transgenic goats for recovery from milk.

Given Data / Assumptions:

• Mammary gland–specific promoters can direct expression of human proteins into milk.
• Milk provides a scalable and relatively noninvasive bioreactor system.
• Downstream purification from milk is well established for recombinant biologics.

Concept / Approach:
Using mammary-specific regulatory elements (for example, beta-casein promoter), transgenes are expressed during lactation and secreted into milk. This enables large-scale production without terminal animal procedures and in a matrix that supports stable protein secretion and collection.

Step-by-Step Solution:

Verification / Alternative check:
Published case studies and regulatory interactions have documented the feasibility of recovering active biologics from goat milk, including tPA variants and other therapeutic proteins.

Why Other Options Are Wrong:

• B: Blood expression is less controllable and has animal welfare and processing drawbacks.
• C/D/E: Urine, muscle, and saliva are not standard, high-yield, mammary-directed secretion routes for tPA.

Common Pitfalls:
Assuming ubiquitous expression is desirable; in practice, tissue-specific expression simplifies purification and minimizes systemic effects.

Final Answer:
Milk