5 Alpha-reductase Deficiency type 2

During the development of a fetus with chromosomes 46,XY, the testes produce androgens (i.e., male hormones) that give genitals typically male characteristics. Specifically, the hormone involved is testosterone, which in turn is transformed into dihydrotestosterone (DHT) by the enzyme 5 alpha reductase 2 (5αR2). DHT is 2–5 times more biologically active than testosterone, and is critical for the development of the prostate and typically male external genitalia (primarily the penis). Due to a genetic mutation, 5αR2 may be defective and unable to produce DHT. As a result, at birth 5αR2 carriers may exhibit external genitalia that are not typically male (such as a small, sometimes clitoris-like, penis and a bifid scrotal sac, reminiscent of labia majora and therefore often called “labioscrotal folds”). The testes are generally located in the inguinal canal or within the labioscrotal folds, and there is a blind vaginal opening, but no uterus. At puberty, some degree of virilization occurs, mainly lowering of the vocal timbre, development of muscle mass, and further development of external genitalia (clitoris/penis), with possible appearance of erections. This happens because, at this stage, testosterone levels are high enough to enable DHT production, even though the enzyme responsible for this conversion is not fully functional.  
Cases of individuals with 5 alpha-reductase deficiency (5αR2D) are most frequently reported in isolated communities, with a high rate of inbreeding, suggesting that this Variation of Sex Characteristics (VSC)/Difference of Sex Development (DSD) is extremely rare in other populations.

Genetic traits
Several types of 5αR2 mutations can give rise to this type of VSC/DSD, and the same genetic mutation can manifest in various forms, which makes it impossible to predict, based on his or her genetics, what the person's characteristics will be. For example, individuals from the same family with the same mutation may have different external genitalia.

Identification
This VSC/DSD is generally recognized clinically at birth or puberty. In the newborn, the external genitalia show traits overlapping with other forms of testosterone synthesis deficiency (especially 17 beta-hydroxysteroid-dehydrogenase type 3 deficiency) and partial Androgen Insensitivity Syndrome (AIS). From puberty onward, you have normal testosterone levels and normal or low DHT levels, with normal or increased testosterone/DHT ratio. For this reason, it is unlikely to obtain an unambiguous picture through hormonal assays alone. Therefore, if 5αR2D is suspected, a genetic investigation is always recommended.

Medical options
Infants with 5αR2D do not have hormonal deficits that may become an immediate health risk. However, gender assignment at birth (which is often based on external genital characteristics) can be quite challenging in some cases. Experts tend to support a registry assignment to the male gender at birth because studies report that about 56-63% of people assigned female at birth (AFAB) have a subsequent change in gender identity/expression from female to male. Considering that only the individual is able to define their own gender identity in the course of their development, the most recent international recommendations support the need to postpone medical interventions, partially reversible or irreversible surgical procedures that are not urgent for the physical health of the child, at a time when the person can express a conscious consent on the matter. 
In the event that the person has a male gender identity and testicular removal surgery (gonadectomy) has not been performed, he does not necessarily have to carry out a hormone therapy during his lifetime, since the testicles are generally functioning from puberty. On the other hand, androgen therapy may become necessary to obtain a complete virilization, especially regarding the development of the external genitalia. Gender affirmation surgical treatments may also be required to optimize the appearance and/or function of the external genitalia, including urination.
In the event that the person has an identity and the desire to affirm a female gender, the goal is to undertake hormonal therapy that simulates the female pubertal development, and then continue with such therapy in adulthood. Some people may wish for surgery to feminize the genitals; such surgery may be necessary to ensure adequate sexual and urinary function. 
If gonadectomy is not carried out, a testicular ultrasound check should be carried out periodically as the risk of testicular cancer is described higher than in the general population, although is not quantifiable.
In the event that a gonadectomy has been performed, it is appropriate that the person effects a hormone replacement therapy (based on male or female hormones, based on perceived gender identity) in order to avoid possible negative health consequences.
Fertility is often impaired and generally people are not able to conceive spontaneously (due to the poor quality of seminal fluid or due to the anatomical characteristics of the external genitalia) but can do so through medically assisted procreation techniques.

Bibliography
Batista RL, Mendonca BB. Integrative and Analytical Review of the 5-Alpha-Reductase Type 2 Deficiency Worldwide. The Applicaion of Clinical Genetics. 2020; 13:83-96
Domenice S, et al. 46, XY Differences of Sexual Development. Last Update: 2022 Aug 21. In: Endotext [Internet] 
Mendonca BB et al. Steroid 5α-reductase 2 deficiency. [Abstract]. The Journal of steroid biochemistry and molecular biology. 2016 Oct; 163:206-11

Further Links
Orphanet. 46,XY disorder of sex development due to 5-alpha-reductase 2 deficiency 
MedLine Plus. 5-alpha reductase deficiency