Schematic summary of the proposed scenario at the edge of the Horsehead nebula. The two crosses indicate the PDR and dense core positions, as in Fig. 2. At the PDR, the moderate FUV field enables the coexistence of C and CO, together with a sufficiently high T_dust to allow efficient diffusion of atomic C on grain surfaces. These conditions may favor the formation of H₂CO, CH₂CO, CH₃CHO, HNCO, and CH₃CN, likely through a combination of gas-phase and grain-surface pathways. Interestingly, for the two species with the lowest PDR abundances, CH₃OH and HC₃N, previous studies suggested ineffective photodesorption and inefficient grain-surface formation, respectively. In the case of the dense core, the FUV-shielded conditions limit CO photodissociation, and the low T_dust makes the diffusion of atomic C on grains inefficient. These conditions may favor the grain-surface formation of H₂CO and CH₃OH, and also contribute to the prevalence of HC₃N in the gas phase as it is less efficiently photodissociated.