Classical conditioning is among the highest-yield topics in behavioral sciences on the MCAT, USMLE Step 1, and nursing licensure examinations. Ivan Pavlov's systematic investigation of conditioned reflexes in the late 19th and early 20th centuries demonstrated a fundamental learning mechanism: organisms can learn to anticipate biologically significant events through environmental signals. Pavlov's observation was deceptively simple — his laboratory dogs began salivating not only when food was presented, but in anticipation of food when signals associated with feeding (lab assistants' footsteps, the sight of food bowls, sounds of food preparation) appeared. This accidental discovery revealed that neutral stimuli acquire biological significance through repeated association. In clinical practice, classical conditioning operates continuously and consequentially. A patient who experienced severe pain during a bone marrow biopsy becomes tachycardic and diaphoretic at the sight of the procedure room on a subsequent visit — before any needle is introduced. A person in recovery from alcohol use disorder experiences intense craving upon driving past the bar where they previously drank. A child who received multiple painful vaccinations cries and pulls away from any nurse in...