**Core Concept**
The underlying condition in this infant is likely an intersex disorder, specifically congenital adrenal hyperplasia (CAH) or androgen insensitivity syndrome (AIS), which are characterized by abnormal gonadal sex development and virilization of female external genitalia.

**Why the Correct Answer is Right**
The first step in diagnosing this condition is to perform a physical examination to assess the degree of virilization, including the presence of clitoral enlargement and labial fusion. This examination will guide further investigations, such as karyotyping, hormonal analysis, and imaging studies. The physical examination will also help to determine the need for immediate medical intervention, such as emergency surgery to correct labial fusion.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography is not the first step in diagnosing this condition, as it may not provide a clear diagnosis and may not be as informative as a thorough physical examination.

**Option B:** Karyotyping is an important diagnostic tool, but it should not be performed as the first step. The physical examination will guide the need for further genetic testing.

**Option C:** Hormonal analysis is crucial in diagnosing conditions such as CAH, but it should not be performed as the first step. The physical examination will guide the need for further biochemical testing.

**Clinical Pearl / High-Yield Fact**
In infants with suspected CAH or AIS, a high index of suspicion is necessary to prevent delayed diagnosis and treatment. A thorough physical examination should be performed at birth, and any abnormalities should be promptly referred to a specialist for further evaluation and management.

**Correct Answer: B. Karyotyping is not the first step, a thorough physical examination is required.**