DNA/RNA/Nucleic AcidsMarch 19, 20264 min read

Everything You Need to Know About Lesch-Nyhan syndrome for Step 1

Deep dive: definition, pathophysiology, clinical presentation, diagnosis, treatment, HY associations for Lesch-Nyhan syndrome. Include First Aid cross-references.

Everything You Need to Know About Lesch-Nyhan Syndrome for Step 1

Lesch-Nyhan syndrome is a classic purine salvage disorder that Step 1 loves because it connects biochemistry → uric acid physiology → neurologic findings → self-injury. If you can recognize its signature triad and understand why it happens, you’ll pick up easy points on both biochem and clinical vignettes.

Where It Fits (Biochemistry → Nucleic Acids)

Purines: Build vs Salvage

Purines (adenine, guanine) come from:

  • De novo synthesis (build from scratch)
  • Salvage pathway (recycle bases from DNA/RNA breakdown)

Lesch-Nyhan = defective salvage.
The key enzyme is HGPRT (hypoxanthine-guanine phosphoribosyltransferase).

Definition (High-Yield)

Lesch-Nyhan syndrome is an X-linked recessive disorder caused by HGPRT deficiency, leading to:

  • Impaired purine salvage
  • Excess de novo purine synthesis
  • Hyperuricemia
  • Neurologic dysfunction + self-mutilation

Classic “buzzwords”:
Self-mutilation, orange sand in diaper, gout, dystonia/choreoathetosis, intellectual disability.

Pathophysiology: The Step 1 “Why”

Normal HGPRT function (salvage)

HGPRT salvages:

  • Hypoxanthine → IMP
  • Guanine → GMP using PRPP (phosphoribosyl pyrophosphate).

What happens when HGPRT is deficient?

  1. ↓ IMP and ↓ GMP (salvage products)
  2. ↑ PRPP (not being used for salvage)
  3. ↓ feedback inhibition of de novo purine synthesis
    • IMP and GMP normally inhibit de novo synthesis; when low → the pathway runs faster.
  4. ↑ de novo purine synthesis → ↑ purine breakdown → ↑ uric acid
  5. Uric acid deposition → gout, nephrolithiasis, renal disease

Testable chain:
HGPRT ↓ → IMP/GMP ↓ → PRPP ↑ + loss of feedback → de novo purines ↑ → uric acid ↑

Clinical Presentation (How It Shows Up on Vignettes)

Hyperuricemia Findings

  • Gouty arthritis (can occur early)
  • Nephrolithiasis (uric acid stones)
  • “Orange sand” (urate crystals) in diaper in infants
  • Potential progression to renal dysfunction

Neurologic + Behavioral Findings (Very High-Yield)

  • Self-injurious behavior (lip/finger biting)
  • Aggression/irritability
  • Dystonia, choreoathetosis, spasticity
  • Developmental delay / intellectual disability

Exam tip: If a vignette mentions self-mutilation + hyperuricemia, Lesch-Nyhan should be your immediate answer.

Diagnosis

Key Labs

  • ↑ Uric acid
  • Evidence of uric acid stones (radiolucent; see below)
  • Sometimes ↑ PRPP (conceptually high-yield; not always given explicitly)

Confirmatory Testing

  • Enzyme assay: low/absent HGPRT activity
  • Genetic testing: mutation in HPRT1 gene

Imaging/Stone Clues

  • Uric acid stones are radiolucent (won’t show on plain X-ray; may be seen on CT/ultrasound depending on context)

Treatment

Manage Hyperuricemia

  • Allopurinol or febuxostat
    • Decrease uric acid production by inhibiting xanthine oxidase
  • Hydration + urine alkalinization (helps prevent uric acid stones)
  • Treat gout flares as appropriate (Step 1 often focuses more on prevention/biochem than flare management)

Address Neurobehavioral Symptoms

  • Supportive, multidisciplinary care:
    • Behavioral interventions, protective devices (to prevent self-harm)
    • Medications for dystonia/spasticity as needed
  • There is no cure that reverses the neurologic phenotype in classic Lesch-Nyhan.

High-yield nuance: Lowering uric acid improves gout/stone complications but does not reliably fix neurologic/self-injury features.

High-Yield Associations & Test Traps

Genetics

  • X-linked recessive
    • Typically affects males
    • Consider carrier status in females

Differential Diagnosis (Know the Contrast)

Kelley-Seegmiller syndrome = partial HGPRT deficiency

  • Hyperuricemia, gout, stones
  • Without severe neuro/self-mutilation seen in classic Lesch-Nyhan

Commonly Tested Connections

  • Purine metabolism disorders → hyperuricemia
  • HGPRT deficiency → purines can’t be salvaged → more de novo synthesis
  • Uric acid stones: classically radiolucent
  • Orotic aciduria is a pyrimidine synthesis disorder (different pathway): don’t confuse with purines.

Step 1 “If You See This, Think Lesch-Nyhan”

  • Male child with self-mutilation (biting lips/fingers) + dystonia/choreoathetosis
  • Infant with orange crystals in diaper
  • Early-onset gout/nephrolithiasis
  • Labs show hyperuricemia
  • Inheritance clue: X-linked recessive

Quick First Aid–Style Summary (Memory Anchor)

Lesch-Nyhan syndrome

  • Cause: HGPRT deficiency (XLR)
  • Biochem: ↓ salvage → ↑ PRPP and ↑ de novo purine synthesis → ↑ uric acid
  • Key findings: self-mutilation, aggression, dystonia/choreoathetosis, intellectual disability, gout, kidney stones, orange sand
  • Tx: Allopurinol/febuxostat, supportive neurobehavioral care

First Aid Cross-References (Where to Review)

In First Aid for the USMLE Step 1, review the sections covering:

  • Purine salvage pathway
  • HGPRT deficiency (Lesch-Nyhan)
  • Xanthine oxidase inhibitors (allopurinol, febuxostat)
  • Gout and uric acid stone concepts These are typically found in the Biochemistry (nucleotide metabolism) and Pharmacology (anti-gout drugs) areas.

Mini-Quiz (1-Step Check)

Q: A young boy has developmental delay, dystonia, aggressive behavior, and bites his fingers until they bleed. He also has hyperuricemia and kidney stones. What enzyme is deficient?
A: HGPRT (hypoxanthine-guanine phosphoribosyltransferase)

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