Differential Diagnosis Endorsed for Abusive Head Trauma: Nevada Affirms Physician Causation Testimony Without Biomechanical Expertise and Clarifies Limits on Juror-Impeachment and Prosecutorial Comment

Introduction

In Melzo (Craig) v. State, No. 87560 (Nev. Sept. 10, 2025), the Supreme Court of Nevada affirmed a first-degree murder conviction stemming from the death of an infant, Zayden Melzo. The case presented a suite of trial and post-trial challenges: voir dire exchanges concerning reasonable doubt; evidentiary rulings on “other-acts” pertaining to both the child’s mother and the defendant; Confrontation Clause and impermissible vouching issues tied to a treating physician’s testimony; scope and reliability of expert testimony diagnosing abusive head trauma (AHT); claims of prosecutorial misconduct in closing argument; and a motion for new trial premised on alleged juror misconduct unveiled by a post-verdict juror survey.

The opinion is most notable for its careful treatment of the evolving science around AHT. While recognizing the modern consensus that the classic “triad” of cerebral edema, subdural hemorrhages, and retinal hemorrhages is not pathognomonic of abuse, the court held that AHT opinions remain admissible and reliable in Nevada when physicians employ a thorough differential diagnosis—testing for and ruling out alternative causes—and base their causation conclusions on clinical treatment and record review. The court also reiterated that physicians need not possess biomechanical engineering credentials to offer causation opinions when their medical foundation is sufficient.

Summary of the Opinion

• Voir dire: The prosecutor’s repeated inquiry distinguishing “beyond a reasonable doubt” from “beyond all doubt” did not plainly err, particularly where the court disclaimed any voir dire definition and later gave the statutory reasonable doubt instruction. (Tucker v. State approved instruction echoed.)
• “Other-acts” evidence:
  • Excluding evidence that the child’s mother used marijuana while pregnant and left the hospital AMA after a fall was proper; no showing tied those acts to the infant’s injury or a permissible non-propensity purpose.
  • Admitting testimony that the defendant strictly controlled feeding was an abuse of discretion due to irrelevance, but harmless because the defendant explained his conduct.
  • A separate, unobjected-to anecdote (leaving the mother at a doctor’s appointment) did not amount to plain error given defendant’s explanatory testimony and lack of prejudice.
• Confrontation Clause: A treating physician’s brief reference to having consulted colleagues did not introduce testimonial statements and, in any event, the court sustained the objection and instructed the jury to disregard—curing any potential error.
• No impermissible vouching: A physician’s explanation that he relied on another doctor’s clinical skill did not amount to vouching for truthfulness.
• Expert scope and reliability:
  • Physicians may opine on causation without biomechanical expertise if they have a sufficient medical foundation (Rish v. Simao; Khoury v. Seastrand; Murphy v. S. Pac. Co.).
  • Given a thorough evaluation that ruled out infection, metabolic disorders, and accidental trauma—and with defense experts heard—AHT testimony was grounded in reliable science.
• Prosecutorial misconduct: The rebuttal did not shift the burden by characterizing defense expert opinions as speculation; commentary on the defense expert’s compensation and calling him a “sad advocate” was borderline but not plain error in context and did not cause actual prejudice.
• Juror misconduct/new trial: Post-verdict juror statements reflecting subjective deliberations were inadmissible under NRS 50.065. The few objectively framed comments (e.g., a juror saying the defendant could “get another trial” and some jurors standing/talking over others) did not show extraneous influence or prejudicial misconduct. Denial of new trial affirmed.

Analysis

Precedents Cited and Their Influence

• Issue preservation and plain error:
  • Lamb v. State and Pantano v. State: Failure to object on specific grounds below limits appellate review to plain error. This principle drove the court’s posture on several issues (voir dire, expert qualification, some misconduct claims).
  • Jeremias v. State, Green v. State, Valdez v. State, and United States v. Olano: Articulate Nevada’s plain error test—error that is clear under current law and affects substantial rights, causing actual prejudice or a miscarriage of justice.
• Reasonable doubt in voir dire:
  • Tucker v. State: Approved language clarifying the State’s burden is not “absolute certainty,” anchoring the court’s conclusion that referencing the distinction between “reasonable doubt” and “all doubt” was not error.
• “Other-acts” evidence:
  • Hubbard v. State and Rhymes v. State: Abuse-of-discretion standard for admitting or excluding other-acts evidence.
  • Bigpond v. State: Nevada’s three-part test for other-acts admissibility: relevance to a non-propensity purpose, clear and convincing proof, and probative value not substantially outweighed by unfair prejudice.
  • Mortensen v. State: The other-acts framework applies to witnesses (not just defendants), guiding exclusion of the mother’s prior conduct.
  • Newman v. State and Tavares v. State: Harmless error requires a “substantial and injurious effect or influence,” which the court did not find regarding the “strict feeding” testimony.
• Confrontation and vouching:
  • Chavez v. State, Crawford v. Washington, and Melendez-Diaz v. Massachusetts: Define testimonial hearsay and the Sixth Amendment’s constraints. The court found no testimonial statements were introduced.
  • Perez v. State and Lickey v. State: Forbid vouching for another witness’s truthfulness; the court distinguished permissible reliance on a colleague’s clinical skill from vouching.
• Expert scope and reliability (causation):
  • Rish v. Simao, Khoury v. Seastrand, Murphy v. S. Pac. Co.: Support allowing medical doctors to opine on causation with an adequate medical foundation, even outside sub-specialties like biomechanics.
  • On evolving AHT science: The court surveyed scholarship (Deborah Tuerkheimer’s article) and cases reopening convictions when alternative causes were not explored or defense evidence was excluded (Allison v. State (Alaska); People v. Bailey (N.Y.); State v. Edmunds (Wis.)). The court emphasized that AHT remains a recognized cause but no longer the only cause of the triad.
• Prosecutorial comments:
  • Valdez v. State: Two-step analysis for prosecutorial misconduct (impropriety and prejudicial effect), overlaid with plain error where no objection.
  • Barron v. State: Reaffirms State’s burden to prove guilt beyond a reasonable doubt.
  • Burns v. State: Prosecutors may attack the plausibility of a defense theory based on the evidence without improperly shifting the burden.
  • Butler v. State, McGuire v. State, and Sipsas v. State: Disapprove disparaging remarks and undue focus on expert compensation. The court found the challenged remarks here borderline but not plainly erroneous or prejudicial.
• Juror misconduct and new trial:
  • Meyer v. State, United States v. Saya, and Tanksley v. State: Denials of new trial reviewed for abuse of discretion; two-part showing required (misconduct occurrence and resulting prejudice).
  • NRS 50.065 and Meyer: Strong bar on juror testimony about deliberations, subjective impressions, or internal influences; courts may consider only objective evidence of extraneous information or outside influence.
  • United States v. Brito and related authorities: Juror comments about sentencing or similar topics within deliberations are ordinarily intrinsic, absent extrinsic inputs.

Legal Reasoning

1) Voir dire and “quantifying” reasonable doubt

Although defense counsel objected to the repetition of the State’s question, no objection was lodged to the substance of asking venire members whether they would hold the State to “beyond all doubt.” Under Nevada’s preservation doctrine, that limited the appellate court to review only for plain error. The court found no error at all: the phrasing tracked language Nevada has approved (that absolute certainty is not required) and the trial judge both disclaimed any voir dire definition and later delivered the statutory reasonable doubt instruction. The court also found no effect on substantial rights.

2) “Other-acts” evidence about the mother (excluded) and about the defendant (partly admitted)

Applying Bigpond, the court affirmed exclusion of evidence that the child’s mother used marijuana during pregnancy and left the hospital AMA after a fall while pregnant. The defense failed to connect those acts to any proper, non-propensity purpose or to the injury mechanism. The acts lacked relevance to the charged crime and were not shown by clear and convincing evidence to bear on a permissible theory like alternative causation.

Conversely, testimony that the defendant strictly controlled feeding was deemed irrelevant to any element of the charged offense and thus admitted in error. Yet the error was harmless under Newman/Tavares because the defendant had a full chance to contextualize and mitigate any negative inference (explaining his feeding concerns). A separate anecdote—that he left the mother at a doctor’s appointment without a vehicle—did not produce plain error because the defendant’s explanatory testimony negated prejudice.

3) Confrontation Clause and vouching by a treating physician

The treating physician briefly referenced consulting “many faculty in the community” (including PICU) regarding cause of death. Because the court sustained the objection immediately and no specific out-of-court testimonial statements or reports were put before the jury, no Confrontation Clause violation occurred under Crawford/Melendez-Diaz. Even if the remark had been problematic, the court’s prompt curative instruction dispelled any prejudice.

On alleged vouching, the physician explained he followed another doctor’s recommendations due to that doctor’s clinical reputation. Nevada forbids vouching for another witness’s truthfulness, but the court drew an important line: explaining professional reliance during treatment is not a comment on veracity. This preserved the permissible scope of foundation testimony for medical decision-making without endorsing the truthfulness of another witness’s trial testimony.

4) Expert scope and the reliability of AHT opinions

The defense argued that only biomechanical experts could opine on mechanism and that AHT is scientifically unsound. Reviewing for plain error, the court reaffirmed that physicians may offer causation opinions when grounded in sufficient medical training and case-specific foundation (Rish; Khoury; Murphy). The treating doctors and experts relied on clinical care and chart review—well within medical expertise.

On the reliability of AHT testimony, the court canvassed the literature and case law charting the discipline’s evolution: the old “triad means abuse” view has receded; the triad can also reflect accidental trauma or underlying medical disorders. But abuse remains a recognized cause. Reliability today turns on rigorous differential diagnosis. Here, clinicians tested for infection and metabolic disorders, explored accidental trauma (both parents denied accidents), and the defense presented its own expert challenging shaking as a cause. In that posture, the State’s AHT opinions were not only within the experts’ scope but also reliably derived from accepted clinical methodology.

5) Prosecutorial misconduct in closing argument

The court applied the Valdez two-step framework under plain error review. First, characterizing defense expert testimony as “mere speculation” did not shift the burden of proof. Prosecutors may attack the reasonableness of defense theories based on the evidence (Burns) and may emphasize that reasonable doubt is not mere conjecture. Second, the State’s remarks about the defense expert’s compensation and calling him a “sad advocate” were “borderline” under Butler/Sipsas, which disapprove disparagement and compensation-focused attacks. But this single, passing comment in a lengthy trial did not amount to egregious, obvious misconduct producing actual prejudice.

The State also argued it did not need to specify the precise injury mechanism so long as the defendant’s conduct, while alone with the child, proximately caused death. The court treated this as permissible explication of the State’s burden rather than burden-shifting—consistent with general homicide principles that the prosecution must prove causation beyond a reasonable doubt, not necessarily recreate the precise biomechanics of the lethal act.

6) Juror misconduct and the new-trial motion

Nevada’s juror-impeachment rule (NRS 50.065) largely tracks its federal counterpart: courts may not consider juror testimony about deliberations, mental processes, or the effect of anything on any juror’s vote. Most of the juror survey responses—expressing doubt, perceived railroading, and internal debate—were inadmissible subjectivities. Two arguably objective observations were insufficient:

• A juror’s statement that another juror mentioned the defendant “could get another trial if [they] wrongfully convicted him” did not show extrinsic information; like sentencing talk, such speculation about legal outcomes remains intrinsic absent an outside source.
• Reports that two jurors stood and talked over others described internal dynamics far short of intimidation or harassment, which are generally not admissible to impeach a verdict.

Without admissible proof of extraneous influence or prejudicial misconduct, the district court did not abuse its discretion in denying a new trial.

Impact

• Abusive Head Trauma litigation:
  • This decision signals Nevada’s practical gatekeeping approach to AHT: admissibility and reliability hinge on thorough differential diagnosis and case-specific clinical analysis, not on biomechanical credentials. Prosecutors should ensure a documented, methodical exclusion of alternative causes; defense should be prepared to present competing experts and concrete alternative etiologies.
  • Appellate challenges to AHT opinions are less likely to succeed when the record shows a robust evaluation of medical and accidental causes and the defense was not hindered in presenting contrary science.
• Expert qualifications:
  • The reaffirmation that physicians can opine on causation without biomechanical specialization lowers the threshold for the State to proceed with treating-physician testimony, while keeping the door open for defense to probe the strength of the medical foundation on cross-examination.
• Confrontation Clause in team-based medicine:
  • Brief, non-specific references to collaborative clinical review typically do not introduce testimonial hearsay. Counsel should avoid eliciting specific out-of-court findings or reports unless the declarant is produced or the statements fall within a firmly rooted exception compatible with the Sixth Amendment.
• Voir dire on reasonable doubt:
  • Prosecutors may test for jurors who improperly demand “absolute certainty,” especially where the judge clarifies that voir dire does not define reasonable doubt and the correct instruction is later given. Defense should object not only to repetition but also to substance if phrasing risks “quantification.”
• Relationship-character evidence:
  • “Control” dynamics in a relationship, absent a direct nexus to the charged crime, are at risk of exclusion as irrelevant or as propensity inferences. Even when admitted, such errors can be harmless if the accused can contextualize the conduct. Practitioners should tie such evidence to a non-propensity theory (motive, opportunity, identity, etc.) and articulate probative value.
• Prosecutorial commentary boundaries:
  • While fair comment on the evidence is permitted, disparaging labels and excessive focus on expert compensation remain disfavored and could warrant reversal if objected to and prejudicial. The “borderline” remarks here survived only under plain error review in a long trial with substantial evidence.
• Post-verdict juror inquiries:
  • Most post-trial juror comments about deliberations will be inadmissible. New-trial motions should concentrate on extraneous information, outside influences, or overt external pressures. Subjective doubts or internal dynamics, standing alone, will not impeach the verdict.
• Preservation matters:
  • Several issues were reviewed only for plain error. Timely, specific objections preserve robust appellate review and may alter outcomes on close questions (e.g., prosecutorial remarks, expert scope).

Complex Concepts Simplified

• Plain error: An unobjected-to error the appellate court may correct only if it is clear under current law and affected substantial rights by causing actual prejudice or a miscarriage of justice.
• Abuse of discretion: A deferential standard; the appellate court upholds the trial court’s evidentiary rulings unless they were arbitrary or capricious or rested on a clearly erroneous view of the law.
• Harmless error (“substantial and injurious effect”): Even if the trial court erred, reversal is unwarranted unless the error likely influenced the verdict in a significant way.
• Other-acts evidence: Evidence of a person’s other conduct (not at issue in the charged event). Admissible only for specific, non-propensity purposes (motive, intent, identity, absence of mistake, etc.), proven clearly and convincingly, and if probative value is not substantially outweighed by unfair prejudice.
• Confrontation Clause/testimonial statements: The Sixth Amendment bars admitting out-of-court testimonial statements against a defendant unless the declarant is unavailable and there was a prior opportunity to cross-examine. “Testimonial” generally means statements made with the expectation of use in prosecution.
• Vouching: A witness (or lawyer) cannot assure the jury that another witness is truthful. However, a physician may explain professional reliance on colleagues’ clinical skills as part of treatment decisions.
• Differential diagnosis: A standard medical process that identifies a condition by systematically considering and ruling out alternative causes based on tests, history, and clinical findings.
• Abusive Head Trauma (AHT): A medical diagnosis concerning head injuries in infants/children that can be caused by abuse, accidental trauma, or medical conditions. The historical “triad” (subdural hemorrhage, retinal hemorrhage, brain swelling) is no longer regarded as uniquely indicative of abuse; context and alternative-cause testing are critical.
• Burden shifting vs. fair comment: The State must prove guilt beyond a reasonable doubt and may critique the plausibility of defense theories based on the evidence. It may not suggest the defense must prove innocence or produce evidence.
• Juror-impeachment rule (NRS 50.065): Bars using juror statements to probe deliberations or mental processes; courts may consider only objective evidence of external influences or extraneous information reaching the jury.
• Proximate cause in homicide: The State must prove the defendant’s conduct was a legal cause of death. It need not always prove the precise biomechanical mechanism if the causal chain from conduct to death is established beyond a reasonable doubt.

Conclusion

Melzo v. State offers a contemporary, pragmatic blueprint for litigating—and reviewing—infant head injury prosecutions in Nevada. The court acknowledged scientific evolution on AHT while maintaining that reliable medical testimony, anchored in differential diagnosis and a sufficient clinical foundation, remains admissible without biomechanical specialization. It simultaneously drew sensible boundaries around Confrontation Clause risks in team-based care, clarified that professional reliance is not vouching, and warned prosecutors away from ad hominem or compensation-centric attacks on defense experts.

On core trial mechanics, the decision reinforces familiar but vital rules: avoid “quantifying” reasonable doubt while permissibly clarifying that absolute certainty is not required; tether other-acts evidence to a proper purpose and concrete relevance; preserve objections to secure full appellate review; and remember that internal juror dynamics and subjective doubts rarely open the door to a new trial. While framed as an affirmance, the opinion meaningfully consolidates Nevada’s approach to complex medical causation, evidentiary balance, and post-verdict jury challenges—and will guide courts and counsel navigating the intersection of evolving science and criminal adjudication.