You are collecting and analyzing the secretions of a gland and you find phospholipids. The gland could be all of the following except:

A. hyaluronic acid + proteoglycan monomers: Hyaluronic acid forms the central stalk (handle of the "toilet brush"), and proteoglycan monomers attach along its length, forming a large aggregate that traps water and gives ground substance its gel-like consistency.

B. reticular fibers + collagen fibers: These are structural protein fibers in connective tissue but are not components of proteoglycan aggregates.

C. chondroblasts and chondroclasts: These are cartilage cells involved in formation (chondroblasts) and resorption (chondroclasts) but not part of the ground substance's molecular structure.

D. collagen fibers + fibroblasts: Collagen fibers are structural; fibroblasts produce extracellular matrix components, but neither directly forms the proteoglycan aggregate.

A. in the medullary cavity of a long bone
In adults, the medullary cavity contains yellow marrow, not red marrow.

B. in the epiphysis of a flat bone
Flat bones (e.g., skull) don’t have epiphyses. This answer is anatomically inaccurate.

C. in the medullary cavity of a flat bone
Flat bones don’t have a true medullary cavity. Red marrow is found in their spongy bone, not in a central cavity.

D. in the epiphysis of a long bone
Red marrow in adults is found in the spongy bone of the epiphyses of long bones and in flat bones like the sternum and pelvis.

E. in the epiphyseal plate of a long bone
The epiphyseal plate is made of cartilage, not marrow. It is involved in growth, not hematopoiesis.