Diagnosis: Melasma (Chloasma)
The most likely diagnosis is melasma (also known as chloasma), which are synonymous terms for the same condition—both answers A and B are correct. This 19-year-old pregnant patient at 23 weeks gestation presenting with irregular brownish patches on the face and neck has the classic presentation of pregnancy-induced melasma 1, 2.
Clinical Reasoning
Why Melasma/Chloasma is the Answer
Melasma and chloasma are the exact same condition—the term "chloasma" specifically refers to melasma occurring during pregnancy, also called "mask of pregnancy" 2.
Pregnancy is a major risk factor for melasma due to hormonal fluctuations, particularly in women of reproductive age 1, 3.
The clinical presentation is pathognomonic: irregular hyperpigmented macules and patches distributed symmetrically on the face and neck, which matches this patient's presentation exactly 3.
Timing is consistent: melasma commonly develops during the second and third trimesters of pregnancy when hormonal changes are most pronounced 1.
Why Other Options Are Incorrect
Tinea nigra (Option C) is a superficial fungal infection that presents as a single brown-to-black macule, typically on the palms or soles—not multiple irregular patches on the face and neck 1.
Lichen planus (Option D) presents as violaceous, polygonal, flat-topped papules with Wickham's striae, not brownish patches. The morphology and distribution are completely inconsistent with this patient's presentation 4.
Key Clinical Pearls
Melasma is a benign, hormone-related skin condition that is extremely common during pregnancy and typically requires only symptomatic management 1.
The condition usually resolves postpartum but can become chronic in some cases, potentially impacting quality of life 5.
UV exposure is a critical cofactor—pregnant patients should be counseled on strict sun protection to prevent worsening of existing lesions 2, 3.
No urgent intervention is needed during pregnancy; this is a cosmetic concern without impact on maternal or fetal morbidity or mortality 1.
Management Approach
Reassure the patient that this is a common, benign condition associated with pregnancy that typically improves after delivery 1.
Recommend strict photoprotection including broad-spectrum sunscreen and sun avoidance, as UV radiation significantly worsens melasma 2, 3.
Defer definitive treatment until postpartum, as most cases resolve spontaneously and treatment options (hydroquinone, retinoids, chemical peels) are either contraindicated or unnecessary during pregnancy 1, 2.
Consider vitamin D supplementation if strict sun avoidance is practiced to prevent deficiency 2.