Which of the following best describes a tissue specimen?

A tissue specimen refers specifically to a sample that involves a segment of living tissue, such as skin or an organ, which is removed for the purpose of analysis. This is often done to diagnose diseases, understand the pathology of a condition, or assess the effect of a treatment. The term "tissue" emphasizes that the sample is composed of cells from a particular area of the body, which is crucial for histological examination and other diagnostic tests.

The other choices represent different types of samples that do not fit the definition of a tissue specimen. For example, microbiological testing involves particular organisms rather than a solid tissue mass, fluid samples from joints or muscles pertain to synovial or muscular fluid not tissue, and blood samples focus on the liquid component of the circulatory system. By definition, a tissue specimen specifically requires the physical extraction of solid tissue for study, which aligns perfectly with the description provided in the correct answer.