The doctrine of res ipsa loquitur is a presumption affecting the burden of producing evidence. Evidence Code section 646, subdivision (b). "As the California Supreme Court recently explained, '. . . certain kinds of accidents are so likely to have been caused by the defendant's negligence that one may fairly say "the thing speaks for itself"' or, in Latin, 'res ipsa loquitur.' The doctrine of res ipsa loquitur, which evolved into a rule to determine whether circumstantial evidence of negligence is sufficient, has been codified as 'a presumption affecting the burden of producing evidence.'" Blackwell v. Hurst (1996) 46 Cal.App.4th 939, 943; Evidence Code section 646, subdivision (b). The res ipsa loquitur doctrine applies in a negligence action if the plaintiff establishes: (1) the accident is of a kind that ordinarily does not occur in the absence of someone's negligence; (2) it is caused by an agency or instrumentality within the exclusive control of the defendant; (3) it is not due to any voluntary action or contribution by the plaintiff. Ybarra v. Spangard (1944) 25 Cal.2d 486, 489, 154 P.2d 687; Blackwell v. Hurst, supra, 46 Cal.App.4th at page 943. When these prerequisites are met, the trier of fact may assume the existence of the presumed fact unless the defendant introduces evidence to the contrary. Blackwell v. Hurst, 46 Cal.App.4th at page 943. Res ipsa loquitur is not a theory of recovery, but a judicial doctrine, which, according to section 646, subdivision (b) of the Evidence Code, creates a "presumption affecting the burden of producing evidence." It therefore does not create a separate cause of action. In California, the doctrine is applicable where: (1) the injury is of a kind which ordinarily does not occur in the absence of negligence; (2) the injury is caused by an instrumentality within the exclusive control of the defendant or group of defendants; (3) the injury was not due to any voluntary action or contribution by the plaintiff." ( Rimmele v. Northridge Hosp. Foundation (1975) 46 Cal. App. 3d 123, 129, 120 Cal. Rptr. 39, quoting from Zentz v. Coca Cola Bottling Co. (1952) 39 Cal.2d 436, 446-447, 247 P.2d 344) A narrow exception to the expert witness requirement exists where "the conduct required by the particular circumstances is within the common knowledge of the layman." ( Flowers v. Torrance Memorial Hospital Medical Center, supra, 8 Cal.4th at p. 1001) This exception applies only in cases where the doctrine of res ipsa loquitur applies, for example, where an X-ray shows that a scalpel was left in the patient's body following surgery. (Id.; Blackwell v. Hurst (1996) 46 Cal.App.4th 939, 943-944; Gannon v. Elliott (1993) 19 Cal.App.4th 1, 6-7.) In every other case, expert testimony is essential. ( Kelley v. Trunk (1998) 66 Cal.App.4th 519, 523.) res ipsa loquitur cases in california A doctor's negligence is not presumed but must be proved, except in cases where the doctrine of "res ipsa loquitur" applies. ( Lashley v. Koerber (1945) 26 Cal.2d 83, 84, 156 P.2d 441.) When an accident or disagreeable consequence is so likely to have been caused by the defendant's negligence, it may fairly be said that "'the thing speaks for itself'" or, in Latin, res ipsa loquitur. ( Brown v. Poway Unified School Dist. (1993) 4 Cal.4th 820, 825, 843 P.2d 624.) "'Res ipsa loquitur applies where the occurrence of an injury is of such a nature that it can be said, in the light of past experience, that it probably was the result of negligence by someone and that defendant probably is the person who is responsible. In determining whether such probabilities exist with regard to a particular occurrence, the courts have relied on both expert testimony and common knowledge.... Ordinarily, the standard of care required of a doctor, and whether he exercised such care, can be established only by the testimony of experts in the field.' Citation." ( Gannon v. Elliot (1993) 19 Cal.App.4th 1, 6; Barris v. County of Los Angeles (1999) 20 Cal.4th 101, 108, fn. 1, 972 P.2d 966 standard of care against which acts of a medical practitioner are to be measured is a matter peculiarly within the knowledge of experts, and presents the basic issue in a malpractice action.) "'But to that rule there is an exception that is as well settled as the rule itself, and that is where "negligence on the part of a doctor is demonstrated by facts which can be evaluated by resort to common knowledge, expert testimony is not required since scientific enlightenment is not essential for the determination of an obvious fact."' ( Friedman v. Dresel (1956) 139 Cal. App. 2d 333, 341, 293 P.2d 488 ..., quoting Lawless v. Calaway, supra, 24 Cal.2d at p. 86 ....)" ( Gannon v. Elliot, supra, 19 Cal.App.4th 1, 6.) Thus, the standard of care in a medical negligence case can be proved only by expert testimony unless the proper conduct required by the doctor in the particular circumstances is within the common knowledge of the layperson. (Flowers v. Torrance Memorial Hospital Medical Center (1994) 8 Cal.4th 992, 1001, 884 P.2d 142.) The common knowledge exception is generally limited to situations in which a layperson can reasonably say, based upon universal logic, that the consequences of the doctor's acts are not the sort that would ordinarily have occurred if the doctor had exercised due care. ( Flowers v. Torrance Memorial Hospital Medical Center, supra, 8 Cal.4th at p. 1001.) "The classic example, of course, is the X-ray revealing a scalpel left in the patient's body following surgery." (Ibid.) Other examples include cases in which the patient "received an injury to a part of the body unrelated to the part being treated. (See Ybarra v. Spangard (1944) 25 Cal.2d 486, 487-489, 154 P.2d 687 ... (shoulder injury during appendectomy); Guilliams v. Hollywood Hospital (1941) 18 Cal.2d 97, 99-100, 102, 114 P.2d 1 ... (broken rib in kidney operation).)" ( Hurn v. Woods (1982) 132 Cal. App. 3d 896, 901-902, 183 Cal. Rptr. 495.) "California decisions state that the common knowledge exception applies if the medical facts are commonly susceptible of comprehension by a lay juror--that is, if the jury is capable of appreciating and evaluating the significance of a particular medical event. ( Cobbs v. Grant (1972) 8 Cal.3d 229, 236, 104 Cal. Rptr. 505, 502 P.2d 1 ...; Franz v. Board of Medical Quality Assurance (1982) 31 Cal.3d 124, 141, 181 Cal. Rptr. 732, 642 P.2d 792 ....) As our high court has succinctly put it: 'Technical knowledge is not requisite to conclude that complications from ... a surgical clamp left in the patient's body ... indicate negligence. Common sense is enough to make that evaluation.'" ( Gannon v. Elliot, supra, 19 Cal.App.4th at p. 7.) In sum, under the doctrine of res ipsa loquitur, it may be proper for the jury to "infer negligence from the happening of the accident itself, if it finds based on common knowledge, the testimony of physicians called as expert witnesses, and all the circumstances, that the injury was more likely than not the result of negligence. ( Bardessono v. Michels (1970) 3 Cal.3d 780, 784, 793, 91 Cal. Rptr. 760, 478 P.2d 480 ....)" ( Gannon v. Elliot, supra, 19 Cal.App.4th at p. 6.)