**Core Concept**
Indeterminate leprosy is an early stage of leprosy characterized by the presence of one or a few hypopigmented macules, often on the face, which may be slightly atrophic and have a poorly defined border. This condition represents a threshold between tuberculoid and lepromatous leprosy, where the immune response is still intact but the bacteria are starting to multiply.

**Why the Correct Answer is Right**
The patient's presentation of a single, hypopigmented macule on the face, particularly from Bihar, a region with a high prevalence of leprosy, raises suspicion for indeterminate leprosy. Leprosy is caused by Mycobacterium leprae, which affects the peripheral nerves and skin, leading to hypopigmentation and loss of sensation. In the indeterminate stage, the bacteria are present but not yet causing significant tissue damage. The macule may be atrophic due to the loss of dermal elastic fibers and collagen.

**Why Each Wrong Option is Incorrect**
**Option A:** Pityriasis alba is a common skin condition in children, characterized by well-defined, hypopigmented patches on the face, but it typically resolves within a few weeks and is not associated with nerve involvement.

**Option C:** Vitiligo is an autoimmune condition that causes loss of pigment-producing cells (melanocytes), resulting in well-defined white patches on the skin. However, it is not typically associated with nerve involvement or the atrophic changes seen in leprosy.

**Option D:** Calcium deficiency can lead to various skin manifestations, including dry skin and hair loss, but it is not associated with the specific clinical features of indeterminate leprosy.

**Clinical Pearl / High-Yield Fact**
Indeterminate leprosy often presents with a single, hypopigmented macule on the face, and early diagnosis is crucial to prevent the progression to more severe forms of leprosy. A high index of suspicion, particularly in regions with high prevalence, and a careful clinical examination are essential for early diagnosis.

**✓ Correct Answer: B. Indeterminate leprosy**