Final answer:
The unusual characteristic of endolymph is its higher than normal concentration of potassium ions (K+), differing significantly from the Na+-rich perilymph surrounding it and other extracellular fluids (Option D).
Step-by-step explanation:
The characteristic of endolymph of the membranous labyrinth that is unusual is that it has a higher than normal concentration of K+ (potassium ions).
Unlike other extracellular fluids, which typically have a higher concentration of sodium (Na+) ions, the endolymph stands out because of its high potassium content.
This is in stark contrast with the perilymph that surrounds the membranous labyrinth, which more closely resembles typical extracellular fluid with higher Na+ levels.
The unique ionic composition of endolymph plays a critical role in the function of the inner ear, particularly in the transduction of sound waves and the maintenance of the body's sense of balance.
Hence, the correct answer is Option D.