To prevail on a medical negligence cause of action, the plaintiff must prove the hospital or treating physician has a duty to act according to an applicable standard of care, a breach of that standard of care, an injury resulting from the breach, and a causal connection between the breach and the injury. See Schorlemer v. Reyes, 974 S.W.2d 141, 147 (Tex. App.-San Antonio 1998, pet. denied). A duty to act on a patient's behalf is triggered by a physician-patient relationship. See St. John v. Pope, 901 S.W.2d 420, 423 (Tex. 1995). Only upon a physician's express or implied consent, does a physician-patient relationship come into being. See id. Where no prior relationship exists, the doctor must take some affirmative step to treat the patient before a relationship can be established. See Ortiz v. Shah, 905 S.W.2d 609, 611 (Tex. App.- Houston [14th Dist.] 1995, writ denied); Lopez v. Aziz, 852 S.W.2d 303, 306 (Tex. App.-San Antonio 1993, no writ). Simply being "on call " at a hospital does not establish a physician-patient relationship. Ortiz, 905 S.W.2d at 611; Fought v. Solce, 821 S.W.2d 218, 220 (Tex. App.-Houston [1st Dist.] 1991, writ denied). Once such a relationship exists, however, the physician then owes the patient a duty to treat him or her with the skills of a trained, competent professional, and a breach of that duty may give rise to a malpractice action. See id.