reasoning
Each subject serves as their own control across the egg and egg-free periods, so time-invariant confounders (genetics, habits, baseline lipids) cancel. Randomized order and double-blinding remove period and expectation effects, and the diets were isocaloric and differed only in the added egg, isolating dietary cholesterol/egg matrix as the manipulated variable. The observed +12% LDL / +9% ApoB with HDL unchanged is therefore a causal within-person response, not a cross-sectional correlation. Caveat on external validity (not on causal internal validity): N=17 lacto-vegetarians with low baseline cholesterol intake (97 mg/day) may show a larger fractional response than omnivores with high baseline intake, so the magnitude may not generalize even though the direction does.
Step 6 verdict
approved / checked. Reconstruction: premises = (a) within-subject design, (b) randomized order + double blinding, (c) isocaloric diets differing only in added egg; conclusion = the LDL/ApoB rise is caused by egg intake. Load-bearing step: a within-person contrast under randomization and blinding leaves egg as the only systematically varying input, so the difference identifies its effect. Traced directly: time-invariant subject confounders cancel by differencing; randomized period order removes secular/period trends; blinding removes expectation effects; isocaloric matching removes energy as an alternative channel. No undercutting defeater survives on internal validity (carryover would be the candidate, but a lipid response with a washout is short-lived and would bias toward the null, not create the observed rise). The generalizability caveat concerns the magnitude’s transportability, not the inference, so it does not weaken the verdict.