(i) Given the cyclist’s severe pelvic pain, inability to pass urine, and the retrograde urethrogram showing contrast limited to the superficial perineal pouch, the most likely structure involved is the bulbar (spongy) urethra. This segment, situated within the superficial perineal pouch, is highly susceptible to "straddle injuries" – a common occurrence when falling onto a hard object like a bicycle's crossbar or the edge of a pothole. Such trauma can rupture the urethra, causing urine extravasation into the superficial perineal space and leading to acute urinary retention.

(ii) The principal structure involved is the male urethra, a conduit approximately 18-20 cm long crucial for both urinary excretion and semen passage. Anatomically, it is divided into four main sections:

1. Pre-prostatic (Intramural) Urethra: A short segment within the bladder wall.
2. Prostatic Urethra: Traversing through the prostate gland.
3. Membranous Urethra: The shortest and narrowest part, piercing the deep perineal pouch and urogenital diaphragm.
4. Spongy (Penile) Urethra: The longest part, encased within the corpus spongiosum of the penis, further subdivided into the bulbar, pendulous, and navicular fossa regions.

(iii) The superficial perineal pouch, a potential space bounded by the perineal membrane superiorly and the superficial perineal (Colles') fascia inferiorly, exhibits distinct contents in males and females, reflecting their unique urogenital anatomy.

In Males: The pouch primarily contains the root of the penis. This includes the two crura of the penis, covered by the ischiocavernosus muscles, and the central bulb of the penis, enveloped by the bulbospongiosus muscle. Critically, the spongy (bulbar) urethra traverses through this bulb. Also present are superficial transverse perineal muscles, and branches of the internal pudendal vessels and nerves.

In Females: The pouch houses the root of the clitoris, comprising its two crura (covered by ischiocavernosus muscles) and the paired bulbs of the vestibule (flanking the vaginal orifice), covered by bulbospongiosus muscles. The greater vestibular glands (Bartholin's glands) are also found here. Similar to males, superficial transverse perineal muscles and branches of pudendal vessels and nerves are present.

The key anatomical difference is the male urethra's presence within the bulbar part of this pouch, whereas in females, the urethra passes superior to this space, and the pouch accommodates structures associated with the vaginal opening and clitoris.