The absence of the right kidney in an adult patient during ultrasonography is most likely due to failure of which embryologic structure?

The absence of the right kidney in an adult patient during ultrasonography is most likely due to a failure of the metanephros. The metanephros is the third stage of kidney development in the embryo and is responsible for forming the functional kidneys that persist into adulthood. Development begins around the fifth week of gestation and continues through the third trimester.

If the metanephros does not develop properly, it can result in conditions such as renal agenesis, which is the complete failure of the kidney to form. This can occur on one side (unilateral renal agenesis) or on both sides (bilateral renal agenesis). Given the question focuses on the adult presence (or rather absence) of the right kidney, the failure of the metanephros to develop properly during embryonic development would lead directly to the absence of that kidney after birth.

In contrast, the other embryologic structures mentioned—mesonephros and pronephros—are transient kidney forms that play a significant role early in development but do not contribute to the formation of the kidneys that will function in adults. The splanchnic mesoderm is involved in various developmental processes, including organ development, but it does not specifically pertain to the formation