A 28-year-old Indian woman presents with progressive darkening of urine over the past 6 months, joint pain in the knees and spine, and dark pigmentation of the sclera and ear cartilage. She reports that her urine turns black when left standing for several hours or when exposed to alkaline conditions. Serum creatinine is 1.2 mg/dL (normal). Urine dipstick is negative for protein and blood, but the urine develops a dark brown-black colour on standing. Which enzyme deficiency is responsible for this clinical presentation?

Explanation

## Diagnosis: Alkaptonuria (Homogentisate Oxidase Deficiency) ### Clinical Presentation **Key Point:** The triad of **dark urine on standing**, **ochronosis** (dark pigmentation of connective tissues), and **arthritis** in an adult is pathognomonic for alkaptonuria. ### Pathophysiology of Alkaptonuria Alkaptonuria results from deficiency of the enzyme **homogentisate 1,2-dioxygenase (homogentisate oxidase)**, which catalyzes the degradation of homogentisic acid in the tyrosine metabolic pathway. #### Tyrosine Degradation Pathway ```mermaid flowchart TD A[Tyrosine]:::outcome --> B[4-Hydroxyphenylpyruvate]:::outcome B -->|4-hydroxyphenylpyruvate dioxygenase| C[Homogentisic acid]:::outcome C -->|Homogentisate oxidase<br/>DEFICIENT IN ALKAPTONURIA| D[Maleylacetoacetic acid]:::outcome D --> E[Fumarylacetoacetic acid]:::outcome E --> F[Acetoacetate + Fumarate]:::outcome C -->|Accumulation & Oxidation| G[Dark pigment<br/>Alkapton]:::urgent G --> H[Ochronosis]:::outcome ``` **High-Yield:** When homogentisic acid accumulates, it undergoes **oxidation and polymerization** in the presence of air and alkaline pH, forming a dark brown-black pigment called **alkapton**. ### Biochemical Features | Feature | Alkaptonuria | Normal | |---------|--------------|--------| | Plasma phenylalanine | Normal | Normal | | Plasma tyrosine | Normal or slightly elevated | Normal | | Urine homogentisic acid | Markedly elevated | Absent | | Urine colour (fresh) | Colourless or pale | Pale yellow | | Urine colour (standing/alkaline) | Dark brown-black | Unchanged | | FeCl₃ test | Negative | Negative | **Clinical Pearl:** The urine darkens **on standing** or when exposed to **alkaline conditions** (e.g., addition of NaOH or ammonia) because homogentisic acid is oxidized to the dark pigment in neutral-to-alkaline pH. Fresh acidic urine may appear colourless initially. ### Clinical Manifestations #### Early Life (Childhood–Early Adulthood) - Dark urine on standing or in diapers - Asymptomatic (often discovered incidentally) - Homogentisic acid excretion in urine #### Late Manifestations (Adulthood, as in this case) 1. **Ochronosis** (dark pigmentation of connective tissues): - Sclera and cornea: dark brown-black pigmentation - Ear cartilage: dark pigmentation - Skin: slate-grey or brown discoloration - Caused by deposition of oxidized alkapton polymers in collagen 2. **Arthropathy** (ochronotic arthritis): - Affects large joints: spine, knees, hips, shoulders - Degenerative joint disease with calcification - Intervertebral disc degeneration and ossification - Caused by alkapton deposition in cartilage and connective tissue - Presents with joint pain, stiffness, and reduced mobility 3. **Cardiac involvement** (rare): - Valve calcification and stenosis - Myocardial involvement ### Inheritance and Epidemiology **Mnemonic: AAAA = Autosomal recessive, Adult-onset ochronosis, Arthritis, Alkapton pigment** - **Autosomal recessive** inheritance - Rare in most populations; higher incidence in certain ethnic groups (Middle Eastern, Mediterranean, Indian) - Often goes undiagnosed until adulthood when ochronosis and arthritis develop ### Diagnostic Confirmation 1. **Urine homogentisic acid**: Elevated (can be quantified by HPLC or mass spectrometry) 2. **Urine darkening test**: Fresh urine + NaOH → dark brown-black colour within minutes 3. **Genetic testing**: Mutations in HGD gene (homogentisate 1,2-dioxygenase) 4. **Imaging**: X-rays show degenerative changes, disc calcification, and osteophytes ### Management - **Symptomatic**: NSAIDs for joint pain, physical therapy - **Dietary**: High-dose vitamin C (1–3 g/day) may reduce homogentisic acid oxidation and slow pigment deposition (controversial efficacy) - **Nitisinone** (NTBC): Inhibits 4-hydroxyphenylpyruvate dioxygenase, reducing homogentisic acid production — emerging therapy with promising results - **Supportive**: Joint replacement for severe arthritis **Clinical Pearl:** Unlike PKU, alkaptonuria does NOT cause intellectual disability or neurological damage because homogentisic acid does not accumulate in the brain to toxic levels. The primary damage is to connective tissues (cartilage, bone, skin). ### Why This Case is Alkaptonuria and Not Other Disorders - **Dark urine on standing** is specific to alkaptonuria (homogentisic acid oxidation) - **Ochronosis** (dark sclera, ear cartilage) is pathognomonic for alkaptonuria - **Adult-onset arthritis** with ochronosis is classic alkaptonuria - **Normal renal function** and **negative proteinuria** rule out tyrosinemia type I - **Normal plasma phenylalanine** rules out PKU [cite:Robbins 10e Ch 5; Harper's Illustrated Biochemistry 31e Ch 27]