KITE, Chief Justice.

[¶ 1] The State of Wyoming ex rel. Wyoming Workers' Safety and Compensation Division (the Division) denied Bennie Johnson's requests for preauthorization of bilateral total knee replacements and payment of other medical bills associated with his knees on the grounds the current condition of his knees was not related to a 1992 work injury. The Medical Commission held a contested case hearing and upheld the Division's decision. During the hearing it admitted into evidence, over Mr. Johnson's objection, three exhibits. We conclude the Medical Commission did not commit prejudicial error by admitting the exhibits and substantial evidence supports its decision. We, therefore, affirm.

ISSUES

[¶ 2] Mr. Johnson presents the following issues for this Court's consideration:

The Division presents the same issues, although they are phrased differently.

FACTS

[¶ 3] Mr. Johnson has a long and complicated history of knee problems. On October 9, 1986, Mr. Johnson filed an injury report stating that a day earlier he fell ten feet while working as a pipe welder and injured his left shoulder. It did not indicate that he injured his knees, and he did not receive any worker's compensation benefits for his knees.

[¶ 4] Several years later, while still working as a welder, Mr. Johnson was seen by orthopedic surgeon Christian Guier, M.D. with complaints of bilateral knee pain. On the intake questionnaire dated September 23, 1992, he stated that he had "bad knees" after falling thirty feet in 1985(sic). Dr. Guier diagnosed him as suffering from bilateral patellar tendonitis and chondromalacia and stated he could not rule out mild medial/lateral meniscopathy in both knees.1 Mr. Johnson sought worker's compensation benefits, and the Division concluded the time of the injury was "September 23, 1992" and "the injury appear[ed] to be work related."

[¶ 5] Dr. Guier performed surgery on both knees in December 1992, including bilateral "arthroscopy and chondral debridement for chondromalacia patellae" and "partial synovectomy."2 Although Mr. Johnson returned to work shortly after the surgeries, within a few months he was again suffering from pain in both knees. Dr. Guier performed a second bilateral knee operation in May 1993. Approximately one week after the surgeries, Dr. Guier stated that he felt Mr. Johnson would "be permanently disabled from this injury and should seek another line of employment. A referral to Vocational Rehabilitation would be appropriate at this time." Mr. Johnson apparently remained off work for the next several months, and Dr. Guier continued to suggest he find a new means of earning a living. Mr. Johnson's knees, particularly the left, continued to bother him, and on December 9, 1993, Dr. Guier performed a third arthroscopy and chondral debridement of Mr. Johnson's left knee.

[¶ 6] On December 23, 1993 (approximately two weeks after the left knee surgery), Mr. Johnson's right leg and knee were seriously injured when he was struck by a vehicle while walking. He was diagnosed with "comminuted right knee lateral tibial plateau fracture with disruption of the anterior cruciate ligament insertion of the lateral meniscus," and Dr. Guier performed surgery the next day.

[¶ 7] Mr. Johnson received temporary total disability (TTD) benefits until the end of April 1995, when he accepted permanent partial disability benefits for both knees. He also received a vocational (loss of earning capacity) award to assist him in retraining for another occupation. He apparently used the award to start a fly fishing business sometime in 1995.

[¶ 8] Even though he changed professions, Mr. Johnson continued to suffer from knee problems. He reinjured his right knee while fishing on December 27, 1995, and underwent additional surgeries on his right leg/knee in 1995 and 1996. The Division denied his request for worker's compensation benefits for the 1996 surgery, and a Medical Commission panel upheld the denial, finding the surgery resulted from the pedestrian/car accident rather than his work injury.

[¶ 9] Mr. Johnson fell on April 11, 1997, injuring both knees and again on April 15, 1997, landing on a rock with his right knee. In 1998, Mr. Johnson received injections in his knees to relieve the pain. He underwent bilateral knee surgeries on November 3, 1998, and Dr. Guier stated the "majority" of Mr. Johnson's problems with both knees related to the work injury. Mr. Johnson fell and injured his right knee again in 1999 and underwent additional injections and another surgery on his right knee in 2000.

[¶ 10] During the 2000s, Mr. Johnson had carpel tunnel release surgery and was treated for knee, back, shoulder and neck problems. Over the lengthy term of his medical treatment, Mr. Johnson took a great deal of pain medication and his doctors expressed concern over his pain medication use. In 2003, Mr. Johnson was seen by a pain specialist, Tuenis Zondag, M.D., who noted he "lives with pain 24/7 and has multiple areas with problems."

[¶ 11] Despite his pain, Mr. Johnson worked in his fly fishing business and also returned to welding in the mid 2000s. Finally, in 2009, Dr. Guier and Mr. Johnson requested preauthorization from the Division to perform total knee replacement on both knees. The Division issued final determinations denying both requests and also denying payment of continued treatment of his knees by Dr. Zondag. The Division concluded the treatment and total knee replacements were not associated with the 1992 work injury. Mr. Johnson objected and the matters were consolidated for hearing. The Medical Commission issued its decision on February 1, 2011, upholding the denial of benefits. Mr. Johnson filed a petition for review with the district court, and the district court affirmed. He then filed a timely notice of appeal to this Court.

STANDARD OF REVIEW

[¶ 12] Judicial review of an agency's decision is governed by Wyo. Stat. Ann. § 16-3-114(c) (LexisNexis 2013):

In accordance with § 16-3-114(c), we review the agency's findings of fact by applying the substantial evidence standard. Dale v. S & S Builders, LLC, 2008 WY 84, ¶ 22, 188 P.3d 554, 561 (Wyo.2008). Substantial evidence means "such relevant evidence as a reasonable mind might accept as adequate to support a conclusion." Bush v. State ex rel. Wyo. Workers' Comp. Div., 2005 WY 120, ¶ 5, 120 P.3d 176, 179 (Wyo.2005) (citations omitted). Findings of fact are supported by substantial evidence if, from the evidence preserved in the record, we can discern a rational premise for the findings. Id.

[¶ 13] With regard to an agency determination that the claimant did not satisfy his burden of proof, we have said:

Dale, ¶ 22, 188 P.3d at 561.

[¶ 14] The agency has discretion in ruling on the admissibility of evidence and we do not interfere unless it abused its discretion. Greene v. State ex rel. Wyo. Board of Chiropractic Examiners, 2009 WY 42, ¶ 9, 204 P.3d 285, 290 (Wyo.2009). Discretion generally means "sound judgment exercised with regard to what is right under the circumstances and without doing so arbitrarily or capriciously. An abuse of discretion [is] said to have occurred only when the decision shocks the conscience of the court and appears to be so unfair and inequitable that a reasonable person could not abide it." Airtouch Comm. Inc. v. Dep't of Revenue, 2003 WY 114, ¶ 55, 76 P.3d 342, 361 (Wyo.2003), quoting Sinclair Oil Corp. v. Wyo. Public Service Comm'n, 2003 WY 22, ¶ 41, 63 P.3d 887, 901 (Wyo.2003) (other citations omitted). See also Clark v. State ex rel. Wyo. Workers' Safety & Comp. Div., 968 P.2d 436, 439 (Wyo.1998).

[¶ 15] Even if an agency errs in ruling on an evidentiary issue, we must consider whether its overall decision was arbitrary and capricious, i.e., whether it prejudiced a party's substantial rights, warranting reversal. Greene, ¶ 11, 204 P.3d at 290. We review an agency's conclusions of law de novo and will affirm if the agency's conclusions are in accordance with the law. Moss v. State ex rel. Wyo. Workers' Comp. Div., 2010 WY 66, ¶ 11, 232 P.3d 1, 4 (Wyo.2010).

DISCUSSION

1. Admission of Evidence

[¶ 16] Mr. Johnson challenges the Medical Commission's admission of three exhibits into evidence over his objection. He claims the exhibits were not admissible because they were irrelevant and unreliable. Wyo. Stat. Ann. § 16-3-108(a) (LexisNexis 2013) addresses evidence in contested cases:

[¶ 17] At the beginning of the contested case hearing, the Medical Commission's hearing officer addressed the parties' proposed exhibits. Mr. Johnson's attorney objected to three of the Division's exhibits as irrelevant and unreliable: 1) a 1997 certificate of lien for sales tax due to the State of Wyoming from Bennie C. Johnson, dba Highland Desert Flies; 2) the first page of a 2005 civil complaint in which Mr. Johnson, doing business as Bennie's Welding and Fabrication, was named as a defendant in a breach of contract and slander case; and 3) an anonymous letter to the Division claiming that Mr. Johnson was taking advantage of the worker's compensation system. The State responded that the exhibits were offered for the following reasons: "[T]he first two, go to credibility and also to work history. The last exhibit ... was received by the Division and became part of the Division file. And to the extent for what it's worth, it's submitted for that purpose." The hearing officer stated the exhibits were of "questionable relevance or persuasive value." Nevertheless, he admitted all three, stating that he would "allow the medical hearing panel to determine what particular weight, if any, is associated with those particular exhibits." The exhibits were never mentioned again, either by the parties at the hearing or in the Medical Commission's decision.

[¶ 18] The first two exhibits pertained to Mr. Johnson's work history. The sales tax lien was directed toward Mr. Johnson's fly fishing business and showed that he was involved in that business in 1997. The civil complaint confirmed that Mr. Johnson was engaged in a welding business in 2005. In fact, Mr. Johnson testified that he worked as a fishing guide in 1997, and he later worked in his own welding business. Mr. Johnson's work history was, undoubtedly, relevant in determining what caused his knee problems and whether they were work related. The admission of the exhibits to establish his work history was, therefore, reasonable.3

[¶ 19] On the other hand, the Division did not demonstrate the anonymous letter is the type of evidence a reasonable person would rely upon in conducting his serious affairs. Compare, Gray v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2008 WY 115, ¶ 15, 193 P.3d 246, 251 (Wyo.2008), stating that while it may not have been error to admit an anonymous statement into evidence at an agency hearing, it could not, alone, provide the basis for a reasonable decision. No witness was questioned about the letter and the only rationale given for its admission was that it was part of the Division file. Without more justification as to the letter's admissibility, the agency erred when it allowed it into evidence.

[¶ 20] Nevertheless, the error is not reversible unless it prejudiced Mr. Johnson, resulting in an arbitrary and capricious decision. See Greene, ¶ 11, 204 P.3d at 290. In resolving the compensability issue, the Medical Commission primarily focused on the medical evidence. The letter was not part of that evidence, and the Medical Commission did not even mention the letter in its decision. In Walton v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2007 WY 46, ¶ 37, 153 P.3d 932, 940-41 (Wyo.2007), we stated that it was improper for the Medical Commission to conduct its own neurological examination of the claimant at the hearing, but, given there was no indication it relied on the examination in reaching its decision, reversal was not required. Here, too, the error was harmless and reversal is not warranted. See Grams v. Environmental Quality Council, 730 P.2d 784, 787 (Wyo.1986).

2. Causation

[¶ 21] The Medical Commission concluded Mr. Johnson failed to prove his need for bilateral knee replacement and continued treatment of his knees was related to the 1992 injury. A worker's compensation claimant has the burden of proving each of the essential elements of his claim by a preponderance of the evidence. Kenyon v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2011 WY 14, ¶ 22, 247 P.3d 845, 851 (Wyo. 2011). "As a part of that burden, the claimant must prove a causal connection exists between a work-related injury and the injury for which worker's compensation benefits are being sought." Dale, ¶ 35, 188 P.3d at 563. The requirement of a causal connection is included in the statutory definition of injury in Wyo. Stat. Ann. § 27-14-102(a)(xi) (LexisNexis 2012):

See also Huntington v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2007 WY 124, ¶ 10, 163 P.3d 839, 842 (Wyo.2007). Thus, to satisfy the causation element, Mr. Johnson was required to show a nexus between the condition for which benefits were sought and an activity at work. Id. Physical conditions associated with the normal aging process are specifically excluded from the definition of injury. Section 27-14-102(a)(xi).

[¶ 22] Mr. Johnson presents two arguments challenging the Medical Commission's decision that he did not meet his burden of proving the 1992 work injury necessitated bilateral knee replacements.4 First, he claims the commission incorrectly ruled in Finding of Fact No. 3 that the 1992 injury to his knees was a single injury rather than a cumulative trauma. Finding of Fact No. 3 states in pertinent part:

[¶ 23] Throughout the proceedings in this case, Mr. Johnson referred to the 1992 injury as a "cumulative trauma" injury, presumably meaning an injury which occurs over a substantial period of time under Wyo. Stat. Ann. § 27-14-603(a) (LexisNexis 1991):

[¶ 24] Dr. Guier's letter to the Division which was referenced in Finding of Fact No. 3 was dated January 5, 1993. In that letter, Dr. Guier did not mention Mr. Johnson's earlier fall but, instead, stated: "Bennie's injuries to both knees were secondary to chronic stress placed on the knees from long-term use in his occupation and kneeling on his knees." As the Medical Commission pointed out in Finding of Fact No. 3, the Division did not describe the injury as "cumulative" or one occurring over a substantial period of time in its 1993 initial review allowing coverage. That factual finding does not, however, represent the Medical Commission's ultimate conclusion. Instead, the Medical Commission's decision repeatedly refers to Mr. Johnson's 1992 injury as involving cumulative trauma. In fact, the commission concluded that Mr. Johnson's 1992 injury necessarily included "all prior knee injury pathology." The Medical Commission's decision correctly described the nature of the injury as reflected in the record.

[¶ 25] Mr. Johnson's second challenge to the Medical Commission's decision is that it erred by rejecting Dr. Guier's opinion that his need for bilateral total knee replacements related to the 1992 compensable injury. The Medical Commission has special medical expertise in worker's compensation cases involving complex medical issues. Hathaway v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2014 WY 12, ¶ 25, 317 P.3d 590, 596 (Wyo.2014), citing Stallman v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2013 WY 28, ¶ 28, 297 P.3d 82, 90 (Wyo.2013). As the fact-finder in a contested case hearing, the Medical Commission must parse available medical records and testimony and determine the weight of the available evidence. Id. This includes weighing of expert medical opinions. The agency "`is entitled to disregard an expert opinion if [it] finds the opinion unreasonable, not adequately supported by the facts upon which the opinion is based, or based upon an incomplete and inaccurate medical history provided by the claimant.'" Watkins v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2011 WY 49, ¶ 25, 250 P.3d 1082, 1090-91 (Wyo.2011), quoting Taylor v. State ex rel. Wyo. Workers' Safety & Comp. Div., 2005 WY 148, ¶ 15, 123 P.3d 143, 148 (Wyo.2005).

[¶ 26] Dr. Guier was Mr. Johnson's orthopedic specialist for many years. His records were admitted into evidence and he was deposed several times over the years. The most recent deposition of Dr. Guier occurred in 2010 as part of the contested case proceeding at issue here. Dr. Guier stated that he started seeing Mr. Johnson in 1992 and explained the nature of his initial complaints as:

Dr. Guier diagnosed Mr. Johnson's knee problems as: "chondromalacia [abnormality in the cartilage] in both patellofemoral joints as well as patellar tendinitis." Although he focused primarily on Mr. Johnson's right knee, he suggested that the 1992 work injury had caused the condition which required both of Mr. Johnson's knees to be replaced.

[¶ 27] The Medical Commission rejected Dr. Guier's opinion on causation because it was inconsistent with testimony he had given in 1999, in which he relied heavily upon a "fall" that occurred in 1992, when no such fall had taken place. The Medical Commission pointed out that Mr. Johnson's fall had actually occurred in 1986, and he had only reported an injury to his shoulder, not to his knees. Dr. Guier's 1999 testimony concerning the onset of Mr. Johnson's knee problems was different than his testimony in 2010:

So, in 1999, Dr. Guier believed that Mr. Johnson had stressed his knees while welding and then had fallen shortly before his first appointment in 1992.

[¶ 28] By the time Dr. Guier's deposition was taken in 2010, Mr. Johnson had informed the doctor that his understanding of the history of his knees problems was mistaken, i.e. that the fall had occurred several years before his first appointment. Consequently, Dr. Guier testified in 2010, that Mr. Johnson had fallen in 1986, which caused the initial damage to his knees and he then further stressed the joints while welding. Under both scenarios, Dr. Guier opined, Mr. Johnson's work activities caused his degenerative knee problems. Because of Dr. Guier's incomplete and shifting understanding of the nature of Mr. Johnson's injury, the Medical Commission concluded that it could not rely on Dr. Guier's opinion as to causation. Given the inconsistencies in Dr. Guier's testimony, it was reasonable for the Medical Commission to discount his opinion. In accordance with our standard of review, when an agency explains its reasons for disregarding certain evidence based upon factors contained in the record, its decision will be sustainable under the substantial evidence test. Dale, ¶ 22, 188 P.3d at 561.

[¶ 29] The inconsistencies in Dr. Guier's explanation of Mr. Johnson's injury were not the only grounds for the Medical Commission's decision to disregard Dr. Guier's testimony and conclude that Mr. Johnson had not met his burden of proof. The commission stated that the initial surgery performed by Dr. Guier in 1992 was a simple "clean up" procedure involving a bilateral knee arthroscopy and chondral debridement for the chondromalacia patellae and a partial synovectomy. It noted that he was released within a few weeks to resume his normal activities. After that, he continued to suffer from pain and Dr. Guier recommended he quit the welding profession, which he did. The Medical Commission found:

The agency further ruled:

[¶ 30] The Medical Commission's rulings are supported by the record. Mr. Johnson's right knee was seriously injured in 1993 in a pedestrian/motor vehicle accident. Although Dr. Guier stated the accident did not precipitate Mr. Johnson's need for replacement of his right knee, several independent medical examinations performed after the accident indicated otherwise. In addition, a different panel of the Medical Commission ruled in 1997 that the condition of his right knee at that time was not due to the 1992 cumulative trauma injury, but was caused by the pedestrian/motor vehicle accident. Based upon this evidence, the Medical Commission stated in the current action: "This Panel ... finds that these advanced degenerative changes in the right knee are entirely consistent with the damage sustained in the pedestrian/motor vehicle accident, and are the cause of the need of the total right knee replacement procedure." (emphasis in original). Substantial evidence supports this finding.

[¶ 31] Without providing many details, Dr. Guier also suggested that Mr. Johnson needed a left knee replacement because of the 1992 injury. Addressing the left knee, the Medical Commission stated that an MRI study completed on November 9, 2009, showed Mr. Johnson's current left knee problem was different from the problem he had in 1992. The commission stated:

The MRI report supports the Medical Commission's conclusion.

[¶ 32] Dr. Guier also did not adequately explain the effect of other factors on Mr. Johnson's knee condition. In particular, the doctor did not thoroughly discuss the ramifications of several falls in the late 1990s and 2000s. In Huntington, ¶ 13, 163 P.3d at 844, we noted that the agency is entitled to disregard a medical opinion when the doctor failed to account for intervening injuries. In addition, Mr. Johnson was obese and the medical witnesses, including Dr. Guier, stated that extra body weight contributes to knee problems.

[¶ 33] To recap, the knee injuries Dr. Guier saw Mr. Johnson for in 1992 were relatively minor. Mr. Johnson underwent a simple surgical procedure to "clean up" the cartilage under his knee caps, and was released to return to work shortly thereafter. Mr. Johnson's right knee was severely injured in 1993 when he was hit by a car. Following Dr. Guier's recommendation, Mr. Johnson quit welding and started a fly fishing business. However, he fell several times while walking along the river, resulting in medical treatment for his knees. For several years in the 2000s Mr. Johnson did not see Dr. Guier, but instead saw pain specialist, Dr. Zondag, who treated him for various areas of pain. Finally in 2009, seventeen years after his initial injury, Dr. Guier requested preauthorization from the Division to perform total knee replacements on both knees. Although Dr. Guier asserted Mr. Johnson's knees needed to be replaced because of the 1992 work injury, his understanding of the timing and nature of Mr. Johnson's injuries shifted over time and he failed to explain how the relatively minor cartilage condition led to the need for total knee replacements. In addition, Dr. Guier's opinions did not sufficiently eliminate the intervening accidents, general aging and/or obesity as the cause of Mr. Johnson's need for bilateral total knee replacements.

[¶ 34] Mr. Johnson's lengthy and complicated history of knee problems made understanding the cause of the condition necessitating his total knee replacements difficult. The Medical Commission's expertise in understanding the medical issues is entitled to respect. Hoffman, ¶ 23, 291 P.3d at 305; Stallman, ¶ 28, 297 P.3d at 90. Its decision to disregard Dr. Guier's testimony and conclude that Mr. Johnson had not met his burden of proof was not against the overwhelming weight of the evidence. The Medical Commission's decision to reject the evidence offered by Mr. Johnson is, therefore, supported by substantial evidence.

[¶ 35] Affirmed.