Q-Bank Breakdown: Erectile dysfunction — Why Every Answer Choice Matters

Erectile dysfunction (ED) questions are classic “Step-style traps”: the stem sounds like a single diagnosis, but the answer choices test whether you can localize the problem (vascular vs neurogenic vs endocrine vs psychogenic vs medication) and choose the most likely cause. The fastest way to stop missing these is to treat every distractor like a mini-lesson—because on the real exam, it is.

Tag: Reproductive > Male Reproductive

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The Q-Bank Vignette (Clinical Style)

A 58-year-old man comes to clinic because of difficulty maintaining an erection for the past 8 months. He has type 2 diabetes mellitus and hyperlipidemia. He used to have regular morning erections but now rarely does. He reports decreased penile rigidity during intercourse and decreased sexual satisfaction. He denies relationship conflict and says libido is “about the same.” Medications include metformin, atorvastatin, and hydrochlorothiazide. Physical exam shows decreased vibratory sensation in both feet. Labs: A1c 9.2%.

Question: What is the most likely cause of his erectile dysfunction?

Answer choices: A. Performance anxiety (psychogenic ED)
B. Decreased nitric oxide–mediated smooth muscle relaxation in the corpora cavernosa
C. Hyperprolactinemia due to pituitary adenoma
D. Low testosterone due to primary testicular failure
E. Peyronie disease due to fibrous plaque formation

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Correct Answer: B. Decreased nitric oxide–mediated smooth muscle relaxation in the corpora cavernosa

This patient has organic ED—suggested by:

• Loss of morning/nocturnal erections (strong clue for an organic cause)
• Longstanding diabetes with peripheral neuropathy signs (decreased vibratory sensation)
• Vascular risk factors (DM, HLD), plus a thiazide (can contribute)

Core physiology (high-yield)

Erection is fundamentally a neurovascular event:

1. Parasympathetic (S2–S4) activation → release of nitric oxide (NO)
2. NO increases guanylate cyclase → $\uparrow cGMP$
3. $\uparrow cGMP$ → smooth muscle relaxation in corpora cavernosa
4. Relaxation → increased arterial inflow + compression of subtunical venules → venous outflow obstruction → erection

Diabetes disrupts this system via:

• Endothelial dysfunction (reduced NO bioavailability)
• Autonomic neuropathy (impaired parasympathetic signaling)
• Accelerated atherosclerosis (reduced arterial inflow)

Exam tie-in: PDE-5 inhibitors

PDE-5 breaks down cGMP. Drugs like sildenafil:

• Inhibit PDE-5 → $\uparrow cGMP$ → improved smooth muscle relaxation and erection
• Require some intact NO signaling (they amplify the pathway; they don’t create it from scratch)

Critical contraindication: nitrates (e.g., nitroglycerin)

• Nitrates increase NO → increases cGMP; PDE-5 inhibitors prevent breakdown → dangerous hypotension

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Why the Other Answers Are Wrong (and What They’re Testing)

A. Performance anxiety (psychogenic ED)

Psychogenic ED is common—but the vignette is steering away from it.

Classic psychogenic features:

• Sudden onset, situational (only with a partner, not during masturbation)
• Normal nocturnal/morning erections preserved
• Often tied to stress, anxiety, relationship issues
• Younger patients more typical (but can occur at any age)

Why it’s wrong here: He lost morning erections, and he has clear organic risk factors (DM with neuropathy).

Pearl: If the stem explicitly says “still has morning erections,” think psychogenic until proven otherwise.

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C. Hyperprolactinemia due to pituitary adenoma

High prolactin suppresses GnRH → ↓ LH/FSH → ↓ testosterone → can cause low libido and ED.

Clues you’d expect:

• Decreased libido is usually prominent
• Symptoms of pituitary mass: headaches, bitemporal hemianopsia
• Galactorrhea (more common in women, but can occur)

Why it’s wrong here: He reports libido is “about the same” and has no pituitary symptoms.

Step tip: Hyperprolactinemia is a “libido first” endocrine cause. If libido is normal and morning erections are gone, think vascular/neurogenic before prolactin.

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D. Low testosterone due to primary testicular failure

Primary hypogonadism = testicular problem → low testosterone with high LH/FSH.

Clues you’d expect:

• Low libido, fatigue, decreased morning erections (can happen)
• Testicular atrophy, infertility
• Gynecomastia (depending on etiology)
• Labs: low T, high LH/FSH

Why it’s wrong here: The vignette emphasizes diabetes complications (neuropathy) and vascular risk. Libido is not notably decreased. Also, primary hypogonadism is usually telegraphed with testicular findings or fertility history.

High-yield nuance:

• Low testosterone can contribute to ED, but it more strongly predicts low libido.
• ED with preserved libido often points to vascular/neurogenic causes.

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E. Peyronie disease due to fibrous plaque formation

Peyronie disease causes penile curvature (often painful), due to fibrous plaques in the tunica albuginea. It can lead to ED—mainly via mechanical difficulty or associated vascular compromise.

Clues you’d expect:

• Penile curvature on erection
• Palpable plaque
• Painful erections early on

Why it’s wrong here: No curvature, pain, or plaque findings are mentioned. This is a distractor testing whether you confuse any penile problem with ED etiology.

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High-Yield ED Localization: A 15-Second Framework

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Medication Pearl (Common Distractor Zone)

Several drugs can contribute to ED, and question writers love this.

Common offenders:

• SSRIs (sexual dysfunction)
• Thiazides
• Beta-blockers (not all equally, but classic association)
• Spironolactone (antiandrogen effects)
• Opioids (↓ GnRH → ↓ testosterone over time)

In this vignette, hydrochlorothiazide may contribute—but the most unifying cause is diabetes-related neurovascular dysfunction.

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USMLE Takeaways (What to Memorize)

• Erection pathway: Parasympathetic → NO → $\uparrow cGMP$ → smooth muscle relaxation → erection
• PDE-5 inhibitors increase cGMP; contraindicated with nitrates (severe hypotension)
• Morning/nocturnal erections preserved = psychogenic
• Diabetes = vasculogenic + neurogenic ED (often with loss of nocturnal erections)
• Low libido points endocrine (low T, high prolactin) more than purely vascular disease
• Peyronie = curvature + plaque, not just “generic ED”