Which scientist is credited with the discovery of the major human blood groups (ABO blood group system)?

Introduction:
Safe blood transfusion is possible today largely because we understand that human blood can be divided into distinct groups based on the presence or absence of specific antigens on red blood cells. This fundamental discovery is associated with a particular scientist whose work transformed clinical medicine and transfusion practice. This question asks you to identify that scientist from among several well known figures in biology and medicine.

Given Data / Assumptions:
- The discovery in question is the human blood groups, specifically the ABO system. - Options list Krebs, Pavlov, Karl Landsteiner and Darwin. - We assume standard historical attributions used in textbooks and exam syllabi. - The focus is on the key person who first described the main blood group system.

Concept / Approach:
Karl Landsteiner, an Austrian immunologist and pathologist, discovered the ABO blood group system in 1900. By mixing blood samples from different individuals, he noticed patterns of agglutination and deduced that there are distinct blood groups (A, B, AB and O). His work explained why some transfusions were successful while others caused severe reactions. For this discovery, he later received the Nobel Prize in Physiology or Medicine. Krebs is known for the citric acid (Krebs) cycle, Pavlov for classical conditioning in dogs and Darwin for the theory of evolution by natural selection. Therefore, Karl Landsteiner is the correct answer.

Step-by-Step Solution:
Step 1: Link the concept of ABO blood groups and safe blood transfusions with early 20th century immunology. Step 2: Recall that Karl Landsteiner identified antigens A and B on red blood cells and corresponding antibodies in serum. Step 3: Recognise that this work led to the classification of blood into A, B, AB and O groups. Step 4: Compare this with the achievements of Krebs, Pavlov and Darwin to confirm they belong to different areas. Step 5: Choose Karl Landsteiner as the scientist associated specifically with discovery of blood groups.

Verification / Alternative check:
Medical and biology textbooks consistently state that Landsteiner discovered the ABO blood group system. Discussions of transfusion history describe how his work reduced the risk of fatal reactions. Nobel Prize records also credit him for this discovery. In contrast, Krebs is always discussed in biochemistry, Pavlov in behavioural psychology and Darwin in evolutionary biology. None of these are credited with blood group discovery. This confirms that Karl Landsteiner is the correct answer.

Why Other Options Are Wrong:
Krebs: Famous for elucidating the citric acid cycle in cellular respiration, not for discovering blood groups. Pavlov: Known for experiments on conditioned reflexes in dogs, forming the basis of classical conditioning in psychology. Darwin: Proposed the theory of evolution by natural selection, which changed biology but is unrelated to blood group classification.

Common Pitfalls:
Because many scientific names are taught together, students sometimes mix up which person did what. A helpful strategy is to pair names with key concepts: Landsteiner with blood groups, Krebs with energy cycles, Pavlov with dogs and bells, and Darwin with evolution. This mental pairing makes it easier to recall the correct association during exams.

Final Answer:
The scientist who discovered the major human blood groups is Karl Landsteiner.