Photon radiation at large transverse momenta at colliders is a detailed probe of hard interaction dynamics. The isolated photon production cross section in deep inelastic scattering was measured recently by the ZEUS experiment, and found to be considerably larger than theoretical predictions obtained with widely used event generators. To investigate this discrepancy, we perform a dedicated parton-level calculation of this observable, including contributions from fragmentation and largeangle radiation. Our results are in good agreement with all aspects of the experimental measurement.PACS numbers: 12.20. Ds, 13.60.Hb, 13.85.Qk Production of final state photons is a classical particle physics measurement, since its cross section can be computed in principle in the well-established framework of Quantum Electrodynamics. In turn, photon production cross sections were suggested as a probe of auxiliary quantities in particle physics, such as the measurement of the gluon distribution in the proton from isolated photon production at hadron colliders. With the advent of precise data on this observable (see [1] for the most recent data), it was realized that the theoretical description of photons produced in reactions also involving final-state hadrons is less straightforward than expected, resulting in extensive theoretical discussion. For isolated photon production at hadron colliders, it turns out that effects from the fragmentation of a hadronic jet into a single, highly energetic photon [2] and effects from the resummation of threshold and recoil corrections [3,4] had to be included to obtain a satisfactory description of experimental data. A very sensitive issue is the definition of isolated photons produced in association with hadrons, since a completely isolated photon is not an infrared safe observable in quantum chromodynamics (QCD). At present, this isolation is usually accomplished experimentally by admitting only a limited amount of hadronic energy inside a cone around the photon direction.Recently, the ZEUS collaboration at DESY HERA reported a measurement [5] of the inclusive production cross section for isolated photons in deep inelastic scattering (DIS). The normalization of the experimentally determined cross section turned out to exceed the cross section expected from the multi-purpose event generator programs HERWIG [9] and PYTHIA [10] by factors 7.9 and 2.3 respectively. Even after normalizing the total event rate, none of these programs was able to describe all kinematical dependencies of the measured cross section. Since the same event generator programs are used frequently to estimate photon production backgrounds for new particle searches in other collider environments, it appears to be very important to determine the origin of these large discrepancies. In a subsequent study [11] the ZEUS measurement was analyzed in view of determining the photon distribution in the proton, relevant for electroweak radiative corrections at colliders.By further analyzing the hadronic final state in isolated phot...