What is a fatty substance that is insoluble in blood called?

The correct answer is lipid. Lipids are a diverse group of organic compounds that are insoluble in water and are primarily hydrophobic or amphiphilic in nature. This means that they do not dissolve well in blood, which is primarily composed of water. Lipids include fats, oils, waxes, and certain vitamins, and they serve various essential functions in the body, such as energy storage, signaling, and serving as structural components of cell membranes.

In contrast, carbohydrates, proteins, and sugars are soluble in blood. Carbohydrates are generally hydrophilic, as they contain multiple hydroxyl groups that interact favorably with water. Proteins, which are made up of amino acids, can also be soluble or insoluble in water depending on their structure and composition. Sugars, being simple carbohydrates, dissolve easily in blood due to their chemical structure, which allows for easy interaction with water.

Thus, lipid is the only category among the options listed that is characterized specifically by its insolubility in blood, making it the appropriate choice for this question.