arXiv:2611.14122v1 Announce Type: new Abstract: Large language models (LLMs) achieve expert-level diagnostic accuracy in static evaluations, where the model receives a curated vignette or case report that already condenses the diagnostically relevant findings. Such evaluations measure inference over already selected evidence; the evaluations do not account for the inquiry used to obtain it. In clinical practice, however, diagnosis is an iterative process of inquiry. Clinicians take the patient's history, or anamnesis (Hx), through targeted questioning, examine the patient, and request and interpret tests, progressively narrowing the differential until they reach a final diagnosis (Dx). We introduce Hx2Dx ARENA, an interactive benchmark of 1'000 clinical cases from the New England Journal of Medicine (NEJM). Each case is a simulated encounter that proceeds from initial presentation through history taking and physical examination to diagnostic workup. We evaluate open- and closed-source LLMs under multiple configurations. Performance varies considerably across models in the interactive setting, and every model loses diagnostic accuracy relative to the static setting, where the full case is presented upfront. The ability to ask the right questions accounts for much of both the gap between models and the loss of accuracy: diagnosis improves as questions become more targeted and build a more complete clinical picture. LLMs thus reason well once the relevant data are available but remain limited in hypothesis-driven information gathering, in knowing what to ask in order to know. Closing this gap is a prerequisite for their safe integration into real-world diagnostic workflows.