**Core Concept**
The discovery of the five basic tastes is a fundamental concept in neuroscience and physiology. The five tastes are sweet, sour, salty, bitter, and umami, which are detected by specialized taste receptors on the tongue and elsewhere in the mouth.

**Why the Correct Answer is Right**
The correct answer is A. Umami because it is the last of the five basic tastes to be discovered. Umami is often referred to as the "fifth taste" and is caused by the binding of glutamate, aspartate, or other amino acids to specific receptors on the tongue. This binding triggers a signaling cascade that ultimately leads to the perception of umami taste. The discovery of umami taste is credited to Kikunae Ikeda, a Japanese chemist who isolated glutamate from seaweed in the early 20th century.

**Why Each Wrong Option is Incorrect**
**Option B:** Sweetness is one of the five basic tastes and was discovered long before umami. Sweetness is mediated by sweet receptors that are activated by sugars and other sweet compounds.
**Option C:** Bitterness is another of the five basic tastes and is mediated by bitter receptors that are activated by bitter compounds. Bitterness was discovered before umami and is an important component of the sense of taste.
**Option D:** Sourness is also one of the five basic tastes and is mediated by sour receptors that are activated by acids. Sourness was discovered before umami and is an important component of the sense of taste.

**Clinical Pearl / High-Yield Fact**
The discovery of umami taste highlights the complex and multifaceted nature of the sense of taste. The five basic tastes work together to give us a rich and nuanced appreciation of the flavors of food and drink.

**✓ Correct Answer: A. Umami**