Left: cutouts (15′′  × 15′′) in all the ALHAMBRA optical/NIR filters for the discovered QSO (highlighted by an open circle). Above each cutout we indicate the filter name and measured magnitude (or 5-σ lower limit). The final image is a colour composite of all bands, where the contrast has been increased to make all objects clearly visible. Images are oriented with north up and east to the left. Right: optical-NIR spectral energy distribution of the discovered QSO. ALHAMBRA photometric detections are represented as circles with associated error bars (arrows indicate the 5σ upper limits). The reference magnitude in the m₈₃₀ filter is indicated by a vertical thick line. The best photo-z template solution (QSO with optical slope index α = −0.25 at z_PHOT = 5.426 ± 0.010) is shown as a green line, while the OSIRIS/GTC spectrum (smoothed with a 7 pixel box) is shown as a thick black line, with the redshift probability function (Pdz) in the inset. The agreement found between the spectro-z and the photo-z is remarkable. The most important emission lines for QSOs at the redshift of the source are also indicated.