USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 35-year-old man presents with a complaint of erectile dysfunction over the last three months. The inability to maintain an erection during sexual activity has put a strain on his relationship. He also lacks morning erections. He was diagnosed with diabetes three years ago, and he has a history of major depressive disorder that is managed with fluoxetine. He takes metformin, and his last HgbA1c was 6.3. What is the most likely cause of his erectile dysfunction?
This disorder is also called impotence and like other sexual dysfunction, this condition becomes more common with age.
Sex can be important within relationships, so erectile dysfunction often carries with it emotional and psychological stigma.
In both males and females,, sexual activity involves a sequence of events called the sexual response cycle.
This cycle has four phases, excitement, plateau, orgasm, and resolution.
In males, this is called an erection.
When these reach the maximum level, it’s called the plateau phase.
Next, the accumulated sexual tension gets released during orgasm, causing ejaculation in males.
Immediately after orgasm comes the resolution phase, where the body slowly returns to its original, un-excited state.
Alright, let’s take a closer look at the penis which is made of three long cylindrical bodies: the corpus spongiosum that surrounds the penile urethra, and the two corpora cavernosa made of erectile tissue.
The corpora cavernosa are wrapped in a fibrous coat called the tunica albuginea, and each corpus cavernosum is made up of blood-filled spaces called the cavernosal spaces.
These areas are lined with endothelial cells surrounded by smooth muscle.
Running down the centre of each corpus cavernosum is a large artery called the deep artery which gives off smaller arteries that supply the cavernosal spaces.
Next, blood get drained from these spaces by small emissary veins, which drain into the deep dorsal vein.
This vein then carries the blood back into the systemic circulation.
Now, the penis receives both somatic and autonomic innervation through the cavernous nerves, which innervate both the corpus spongiosum, and the corpora cavernosa.
You can remember the functions of these fibers with the mnemonic “point and shoot.”
“Point” is erection and it’s caused by “P,” or parasympathetic fibers.
“Shoot” is ejaculation and the “S” is for sympathetic.
Now, an erection can happen in two ways, either by physical stimulation of the penis or genitals, called reflex erection, or by becoming emotionally stimulated by a thought, called psychogenic erection.
The acetylcholine bind to muscarinic receptors on endothelial cells, which activates the enzyme nitric oxide synthase.
Nitric oxide synthase converts the amino acid arginine into citrulline and nitric oxide.
The nitric oxide diffuses into the nearby smooth muscle cells, and activates guanylate cyclase, which converts GTP molecules to cGMP.
This leads to a fall in intracellular calcium levels causing the smooth muscles to relax, and allowing the cavernosal spaces to expand and fill with blood.
With more blood coming in but very little blood leaving, the penis can maintain an erection.
Erectile dysfunction leads to an inability to develop and maintain a full erection.
Organic causes can be divided depending on which step in erection is impaired.
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- "Physiology of Penile Erection and Pathophysiology of Erectile Dysfunction" Urologic Clinics of North America (2005)
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