| RAPD | relative afferent pupillary defect |
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| RBA | relative binding affinity; rescue breathing apparatus; right basilar artery; right brachial artery; ... |
| RBD | recurrent brief depression; relative biological dose; right border of dullness |
| RBE | relative biological effectiveness |
| RBRVS | resource-based relative value scale |
| external aperture of vestibular aqueduct | The external opening of the vestibular aqueduct on the posterior surface of the petrous part of the temporal bone near the groove for the sigmoid sinus. Synonym: apertura externa aqueductus vestibuli. (05 Mar 2000) |
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| laryngeal aperture | The aperture between the pharynx and larynx, bounded by the superior edges of the epiglottis (anteriorly), the aryepiglottic folds (laterally), and the mucosa between the arytenoids (posteriorly). Synonym: aditus laryngis, laryngeal aperture. (05 Mar 2000) |
| lateral aperture of the fourth ventricle | One of the two lateral openings of the fourth ventricle into the subarachnoid space at the cerebellopontine angle. Synonym: apertura lateralis ventriculi quarti, foramen lateralis ventriculi quarti, foramen of Key-Retzius, foramen of Luschka, Retzius' foramen. (05 Mar 2000) |
| frontal sinus aperture | One of a pair of openings in the floor of the frontal sinuses in the nasal part of the frontal bone, through which the frontal sinuses communicate with the ethmoidal infundibulum via the frontonasal duct. Synonym: apertura sinus frontalis. (05 Mar 2000) |