Q-Bank Breakdown: Nucleotide synthesis (de novo vs salvage) — Why Every Answer Choice Matters

You’ve done 1,000 Qs and still somehow “de novo vs salvage” keeps showing up to humble you—usually with a weird anemia, a kidney stone, or a kid with developmental delay. The trick isn’t memorizing one enzyme; it’s recognizing which tissues depend on salvage, what accumulates when a pathway is blocked, and how common drugs exploit these choke points. Let’s break it down the way real Q-banks test it: a clinical vignette, the correct answer, then every distractor and why it’s tempting.

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The Vignette (Q-Bank Style)

A 3-year-old boy is brought in for evaluation of developmental delay and recurrent self-injurious behavior (biting lips and fingers). He has a history of orange “sand-like” crystals in his diapers. Physical exam shows dystonia and choreoathetosis. Laboratory studies reveal hyperuricemia. Which of the following enzyme deficiencies best explains this condition?

Answer choices

A. Adenosine deaminase (ADA)
B. Carbamoyl phosphate synthetase II (CPS II)
C. HGPRT
D. IMP dehydrogenase
E. PRPP synthetase (overactivity)
F. Thymidylate synthase

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The Correct Answer: C. HGPRT deficiency (Lesch–Nyhan syndrome)

Why HGPRT fits this vignette

This is classic Lesch–Nyhan syndrome, caused by deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), a key enzyme in purine salvage.

HGPRT normally:

• Salvages hypoxanthine → IMP and guanine → GMP
• Uses PRPP as the ribose-phosphate donor

When HGPRT is deficient:

• Purines can’t be salvaged, especially in tissues that like salvage (notably the brain)
• Hypoxanthine and guanine get shunted to degradation → uric acid
• PRPP rises, because it’s not being consumed by salvage
• Elevated PRPP drives de novo purine synthesis, which paradoxically creates more purines to ultimately degrade → even more uric acid

High-yield clinical features (USMLE favorites)

• Self-injury (biting), aggression
• Dystonia, chorea, developmental delay
• Hyperuricemia → gout, nephrolithiasis
• Orange crystals in diaper (urate)
• X-linked recessive

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De Novo vs Salvage: The Testable Core

Purine synthesis overview (know the “big picture”)

Purines (A, G):

• Built on the ribose (PRPP is the scaffold)
• De novo pathway starts with PRPP → commits via glutamine-PRPP amidotransferase
• End product IMP, then branches:
  • IMP → AMP (uses aspartate, $GTP$)
  • IMP → GMP (uses glutamine, $ATP$; needs IMP dehydrogenase first)

Salvage (purines):

• Saves energy by recycling bases
• HGPRT salvages hypoxanthine and guanine
• APRT salvages adenine → AMP (less tested, but real)

Pyrimidines (C, U, T): contrast

• Built before being attached to ribose
• Commit step: CPS II (cytosolic)

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Why Each Distractor Is Wrong (and Why It Was Tempting)

A. Adenosine deaminase (ADA) deficiency

Temptation: “Severe genetic disease + pediatric presentation = SCID?”
Reality: ADA deficiency causes SCID due to buildup of deoxyadenosine → increased dATP, which inhibits ribonucleotide reductase → decreased DNA synthesis.

What you’d see instead:

• Recurrent infections (bacterial, viral, fungal)
• Chronic diarrhea, thrush
• Absent thymic shadow
• Low T cells (and often B/NK effects depending on subtype)

Key separation from Lesch–Nyhan:

• SCID is immune failure; not hyperuricemia + self-injury.

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B. Carbamoyl phosphate synthetase II (CPS II) deficiency

Temptation: “Nucleotide synthesis enzyme deficiency → neuro issues?”
Reality: CPS II is the rate-limiting enzyme of de novo pyrimidine synthesis (cytosolic). But the vignette is screaming purine salvage with hyperuricemia.

High-yield anchor: CPS II

• Cytosolic
• Uses glutamine as nitrogen source
• Makes carbamoyl phosphate for pyrimidines
• Regulated by:
  • Inhibited by UTP
  • Activated by PRPP

What CPS II issues would suggest:

• Impaired pyrimidine synthesis (not a classic USMLE congenital syndrome like Lesch–Nyhan)
• Not classically associated with hyperuricemia

Common trap: confusing CPS II with CPS I (urea cycle, mitochondria, hyperammonemia).

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D. IMP dehydrogenase deficiency (or inhibition)

Temptation: It’s in the purine pathway and sounds important.
Reality: IMP dehydrogenase converts IMP → XMP → GMP (a step toward guanine nucleotides). Deficiency is not the classic picture here; inhibition is clinically relevant.

High-yield pharm tie-in

• Mycophenolate mofetil inhibits IMP dehydrogenase
• Mainly affects B and T cells, which rely heavily on de novo purine synthesis (less salvage capacity)

What you’d see with inhibition:

• Immunosuppression (transplant, lupus nephritis)
• Not self-injury + hyperuricemia

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E. PRPP synthetase overactivity

Temptation: Also causes hyperuricemia, so it’s a very “mean” distractor.
Reality: This is a real cause of gout due to increased de novo purine synthesis, but it doesn’t explain the neurobehavioral phenotype (self-injury) that points to HGPRT deficiency.

Mechanism:

• PRPP synthetase overactivity → increased PRPP
• Increased PRPP drives glutamine-PRPP amidotransferase → increased de novo purines → increased uric acid

How it presents (classic USMLE framing):

• Early-onset gout, uric acid stones
• Sometimes neuro issues can occur, but Lesch–Nyhan’s self-mutilation is the signature clue for HGPRT deficiency.

Exam takeaway:

• Hyperuricemia alone can be HGPRT deficiency or PRPP synthetase overactivity.
• Self-injury + dystonia strongly favors HGPRT deficiency.

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F. Thymidylate synthase deficiency (or inhibition)

Temptation: Nucleotide synthesis + anemia is often tested, and thymidylate synthase is famous.
Reality: Thymidylate synthase converts dUMP → dTMP (for DNA). Inhibition causes impaired DNA synthesis → megaloblastic anemia, not hyperuricemia with self-injury.

High-yield pharm tie-ins

• 5-FU inhibits thymidylate synthase (via $FdUMP$)
• Methotrexate inhibits dihydrofolate reductase (reduces THF regeneration)
• Trimethoprim/pyrimethamine affect folate metabolism in microbes/parasites

What you’d see instead:

• Macrocytic anemia, hypersegmented neutrophils
• Rapidly dividing tissue toxicity (GI, marrow)
• Not uric acid “sand” in diapers

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The High-Yield Table: Purine vs Pyrimidine (De Novo vs Salvage)

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Rapid-Fire High-Yield Pearls (What Step Loves)

1) Why the brain cares about salvage

Some tissues (notably CNS) rely more on salvage to conserve energy. When HGPRT is absent, the CNS is hit hard → movement disorders + behavioral dysregulation.

2) PRPP is a “gas pedal”

High PRPP pushes:

• De novo purine synthesis forward
• CPS II activation (pyrimidines)

So in HGPRT deficiency: PRPP accumulates → de novo purines increase → uric acid increases.

3) Orotic aciduria vs OTC deficiency (common crossover)

Not this vignette, but commonly paired with nucleotide synthesis:

• UMP synthase deficiency → ↑ orotic acid, megaloblastic anemia, normal ammonia
• OTC deficiency → ↑ orotic acid + hyperammonemia

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How to Answer This in 10 Seconds on Test Day

When you see:

• Self-injury
• Dystonia/chorea
• Hyperuricemia / orange crystals

Think: HGPRT deficiency → Lesch–Nyhan.

If it’s just gout/hyperuricemia without the neurobehavioral hallmark, keep PRPP synthetase overactivity on your differential.