A young man with a predisposition for baldness, and a recent history of psoriasis was suggested to use the following:

Question

A young man with a predisposition for baldness, and a recent history of psoriasis  was suggested to use the following:

BlackTom AI · Accepted Answer

Hair fiber strength depends on sulfur-containing amino acids: cysteine (and its oxidized dimer cystine) forms disulfide (S–S) cross-links in keratin, giving hair its tensile strength and structural integrity. Cysteine contains a thiol (–SH) group; two cysteine residues oxidize to form cystine (R–S–S–R). This chemistry is central to hair and nail resilience, so supplementing cystine/cysteine is the logical choice for someone predisposed to baldness and with recent skin/keratin disturbance such as psoriasis.

Step-by-step reasoning:
1. Hair composition and chemistry: Hair keratin is rich in cysteine residues. Disulfide bonds between cysteines stabilize keratin filaments. Chemically: 2 R–SH → R–S–S–R + 2H+ + 2e– (oxidation) — formation of cystine bridges.
2. Predisposition for baldness (and weakening hair): Increasing available cysteine can support keratin synthesis and disulfide bond formation, potentially improving hair strength and reducing breakage that may mimic or worsen hair thinning.
3. Psoriasis connection: Psoriasis involves abnormal keratinocyte turnover and altered keratinization. Providing cysteine may help normalize keratin synthesis and support repair of the epidermal/appendageal keratin structures.
4. Clinical choice: Cystine/cysteine supplements (often combined with other nutrients) are commonly recommended in dermatologic practice to support hair and nail health.

Why the correct option is a. cystine/cysteine:
- Direct biochemical role in keratin disulfide cross-links makes cystine/cysteine the most relevant supplement for hair-strengthening and keratin-related skin conditions.

Why the other options are incorrect:
- Lysine: important for collagen and general protein synthesis but not a primary sulfur donor for keratin disulfides.
- Methionine: a sulfur-containing essential amino acid that can be a precursor to cysteine via transsulfuration, but it is less directly involved in disulfide bonding than cysteine/cystine and is not the standard direct supplement for hair.
- DL-phenylalanine: a precursor to neurotransmitters, unrelated to hair keratin disulfide formation.
- Tyrosine: precursor for melanin (pigmentation) and some neurotransmitters, not primarily for keratin cross-linking.

Therefore, the correct answer is a. cystine/cysteine, matching the provided answer.

A young man with a predisposition for baldness, and a recent history of psoriasis was suggested to use the following:单项选择题

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