The prominent method to teach clinical reasoning in preclinical years is case-based discussions. It necessitates hours of sessions. A randomised-controlled study has been conducted to determine whether preclinical students can develop illness scripts only by taking online formative testlets. A pre-test-post-test randomised-controlled design has been utilised. The intervention group (N = 20) answered general surgery questions for 18 days, while the control group (N = 19) received questions regarding urinary tract infections. These testlets were comprised of ContExtended Questions, which is a web-based tool to teach clinical reasoning. The performance was assessed by using Key-Feature Questions. While post-test scores were significantly higher than pre-test scores in the intervention group (p < 0.001), there was no statistically significant difference between the pre-test and post-test scores in the control group (p = 0.30). Taking formative case-based multiple-choice testlets without any additional teaching could assist preclinical students to learn clinical reasoning.