Decarboxylation is an important process in biochemistry, so you definitely want to devote a bit more attention to this process than may have been required in your organic chemistry course. For the MCAT, detailed prior knowledge will not be as important as a comfortable familiarity.

While the decarboxylization of both α and β-keto acids occurs in respiration, the decarboxylization of β-keto acids is simpler to achieve in the organic chemistry laboratory. In the citric acid cycle, the oxidation of Isocitrate by NAD+ (by converting a hydroxyl group to a carbonyl group) produces a β-keto acid leading to decarboxylization to form α-Ketoglutarate. The reverse of the decarboxylization of a β-keto acid occurs in photosynthesis with the carboxylization of the enediol intermediate of ribulose 1,5-bisphosphate to form a β-keto acid as part of fixing CO2. Obviously, the last bit about photosynthesis is a comprehension exercise in the context of the MCAT. Plants are not an MCAT topic.