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The main characteristic of 5alpha-reductase deficiency in genetic males is the inability to convert testosterone to dihydrotestosterone (DHT). DHT is a potent androgen that plays a crucial role in the development of male external genitalia and secondary sexual characteristics during puberty. In individuals with 5alpha-reductase deficiency, although testosterone levels may remain normal, the conversion to DHT is impaired.

As a result, affected individuals often present with ambiguous genitalia at birth or underdeveloped external genitalia, while internal genital structures such as testes remain present. During puberty, the presence of normal testosterone often leads to increased virilization, but the lack of DHT results in incomplete or impaired development of male characteristics. Hence, DHT deficiency has a significant impact on the phenotypic expression of male traits.

The other options describe hormonal changes that do not primarily characterize 5alpha-reductase deficiency. These alternatives suggest alterations in hormonal levels, which are not as directly linked to the core issue of the inability to convert testosterone to DHT. Therefore, option D precisely identifies the main pathological feature of this condition.