Karl Landsteiner, an Austrian immunologist and pathologist, is best known for discovering which fundamental feature of human blood?

Introduction:
Karl Landsteiner is one of the most important scientists in medical history because his work made safe blood transfusions possible. Before his discovery, transfusions were extremely risky due to unknown incompatibilities. This question asks you to identify the specific aspect of human blood that Landsteiner discovered and for which he later received the Nobel Prize in Physiology or Medicine.

Given Data / Assumptions:
- The scientist mentioned is Karl Landsteiner. - The options include blood corpuscles, blood groups, blood vessels and blood proteins. - We assume standard historical facts about his contribution to transfusion medicine. - The task is to match his name with the correct discovery.

Concept / Approach:
Karl Landsteiner discovered the ABO blood group system in 1900. He demonstrated that human blood can be classified into different groups based on the presence or absence of specific antigens on red blood cells and corresponding antibodies in the plasma. This explained why some transfusions were successful and others failed catastrophically. His discovery of blood groups, rather than blood cells in general or vessels, revolutionised transfusion practice. Later, he was also involved in the discovery of the Rh factor, but his primary and most famous contribution is the ABO blood group classification.

Step-by-Step Solution:
Step 1: Recall that Landsteiner is associated with safe blood transfusions and classification of blood into groups A, B, AB and O. Step 2: Recognise that the term blood corpuscles (red and white cells) and blood vessels were known long before his time. Step 3: Understand that his Nobel Prize in 1930 was specifically for the discovery of human blood groups. Step 4: Look at the answer options and identify the one that mentions major human blood groups or ABO system. Step 5: Choose the option stating that he discovered the major human blood groups as the correct answer.

Verification / Alternative check:
Medical history accounts describe how Landsteiner mixed blood samples from different individuals and observed agglutination patterns. From this research, he deduced that people belong to distinct blood groups and that incompatibility causes clumping and hemolysis during transfusions. These findings are the foundation of modern transfusion practices and blood banking. The existence of red blood cells and vessels had already been established centuries earlier by other scientists. Thus, all reputable sources confirm that Landsteiner is famous for discovering the major human blood groups, not for basic structural discoveries about blood components.

Why Other Options Are Wrong:
The existence of blood corpuscles: Red and white blood cells were observed much earlier using microscopes and were not Landsteiner's original discovery. The structure of blood vessels: The anatomy and circulation of blood vessels were described by scientists like William Harvey long before Landsteiner. All the proteins present in blood plasma: Proteomics is a much more recent field, and no single scientist, including Landsteiner, is credited with discovering all blood proteins.

Common Pitfalls:
Students sometimes confuse Landsteiner's work on ABO blood groups with later discoveries of the Rh factor or other antigen systems. Another error is to associate his name vaguely with blood research and not recall the specific achievement. To avoid this, remember the phrase Landsteiner and blood groups together and link it with the classification A, B, AB and O, which you often see on blood donation cards and medical records.

Final Answer:
Karl Landsteiner is best known for discovering the major human blood groups (ABO system).