A 42-year-old male presents to the emergency department with acute onset of severe pain, swelling, and erythema of the right big toe that started 6 hours ago. He reports a history of similar episodes over the past 2 years, each lasting 7–10 days. On examination, the first metatarsophalangeal joint is warm, tender, and markedly swollen. Serum uric acid is 8.2 mg/dL (normal <6.8). Synovial fluid analysis shows needle-shaped, negatively birefringent crystals. Which enzyme deficiency is the PRIMARY metabolic basis for this patient's recurrent attacks?

Explanation

## Pathophysiology of Gout and Purine Metabolism ### Clinical Presentation Analysis The patient presents with classic acute gout: sudden-onset monoarticular arthritis of the first MTP joint, with needle-shaped, negatively birefringent monosodium urate (MSU) crystals in synovial fluid. The elevated serum uric acid (8.2 mg/dL) and recurrent nature confirm hyperuricemia-driven gout. ### Why HGPRT Deficiency is the PRIMARY Metabolic Basis **Key Point:** The question asks for the PRIMARY **enzyme deficiency** underlying recurrent gout. HGPRT (Hypoxanthine-Guanine Phosphoribosyltransferase) deficiency is the classic enzyme deficiency that causes hyperuricemia and gout through a well-defined mechanism: 1. **Normal role of HGPRT:** Salvages hypoxanthine and guanine back to IMP and GMP, respectively, consuming PRPP in the process. 2. **When HGPRT is deficient:** The salvage pathway fails → hypoxanthine and guanine cannot be recycled → they are degraded to uric acid → hyperuricemia. 3. **Secondary effect:** Unused PRPP accumulates and drives **increased de novo purine synthesis**, further amplifying uric acid production. 4. **Partial HGPRT deficiency (Kelley-Seegmiller syndrome):** Presents as isolated hyperuricemia and gout in adult males WITHOUT the severe neurological features of complete deficiency (Lesch-Nyhan syndrome). This fits the 42-year-old male with recurrent gout and no neuropsychiatric symptoms. ### Purine Metabolism: Key Enzyme Roles | Enzyme | Effect on Uric Acid | Clinical Phenotype | |---|---|---| | **HGPRT ↓ (partial)** | ↑ Production (salvage pathway blocked + ↑ PRPP → ↑ de novo) | Kelley-Seegmiller: adult gout, hyperuricemia, no neuro features | | **HGPRT ↓ (complete)** | ↑↑ Production | Lesch-Nyhan: gout + dystonia + self-mutilation + intellectual disability | | **PRPP synthetase ↑** | ↑ Production (de novo pathway) | Regulatory overactivity — NOT an enzyme deficiency | | **Xanthine oxidase ↓** | ↓ Production (final step blocked) | Xanthinuria, hypouricemia — opposite of gout | | **Adenosine deaminase ↓** | Accumulation of adenosine/deoxyadenosine | SCID (immunodeficiency), NOT hyperuricemia | ### Why Not the Other Options? - **Option A – Adenosine deaminase deficiency:** Causes SCID (severe combined immunodeficiency) due to accumulation of toxic deoxyadenosine in lymphocytes. It does not cause hyperuricemia or gout. - **Option C – PRPP synthetase overactivity:** This is a **regulatory overactivity (gain-of-function)**, not an enzyme deficiency. The question stem explicitly asks for an "enzyme deficiency." PRPP synthetase overactivity is an X-linked condition causing overproduction gout, but it is not a deficiency and therefore does not satisfy the question's specific wording. - **Option D – Xanthine oxidase deficiency:** This would PREVENT uric acid formation (xanthine cannot be converted to uric acid), causing xanthinuria and hypouricemia — the exact opposite of this patient's presentation. ### High-Yield Summary **High-Yield:** HGPRT deficiency is the classic enzyme deficiency taught in biochemistry as the metabolic basis of gout. Partial deficiency → Kelley-Seegmiller syndrome (adult gout, no neuro). Complete deficiency → Lesch-Nyhan syndrome (gout + neuropsychiatric features). This patient's isolated gout at age 42 without neurological symptoms is consistent with partial HGPRT deficiency (Kelley-Seegmiller syndrome). ### Clinical Pearl **Clinical Pearl:** When a question asks for the "enzyme deficiency" causing gout, the answer is HGPRT deficiency. PRPP synthetase overactivity is a gain-of-function abnormality, not a deficiency, and would not satisfy the question's specific wording. Always read the stem carefully — "deficiency" vs. "overactivity" is a critical distinction in purine metabolism questions. *Reference: Harrison's Principles of Internal Medicine, 21st ed., Chapter on Gout and Hyperuricemia; Robbins & Cotran Pathologic Basis of Disease, 10th ed.*