**Core Concept**
Tricyclic antidepressants (TCAs) are a class of medications used to treat depression, anxiety, and certain other conditions. They work by modulating the levels of neurotransmitters such as serotonin and norepinephrine in the brain. However, TCAs have several potential side effects and contraindications due to their pharmacological properties.

**Why the Correct Answer is Right**
TCAs are generally contraindicated in patients with certain medical conditions or who are taking specific medications. The correct answer is the option where TCAs are not contraindicated.

* **Option A:** Angle-closure glaucoma - TCAs are contraindicated in angle-closure glaucoma because they can increase the intraocular pressure, which can worsen the condition.
* **Option B:** Urinary retention - TCAs can cause urinary retention as a side effect, so they are contraindicated in patients with urinary retention.
* **Option C:** Myasthenia gravis - TCAs are not contraindicated in myasthenia gravis. In fact, some TCAs, such as amitriptyline, can be used to treat the symptoms of myasthenia gravis.
* **Option D:** Heart block - TCAs are contraindicated in heart block because they can slow the heart rate and worsen the condition.

**Clinical Pearl / High-Yield Fact**
TCAs can be used to treat myasthenia gravis, but they should be used with caution and under close monitoring. Amitriptyline is a TCA that has been used to treat myasthenia gravis, and it has been shown to improve muscle strength and reduce symptoms.

**Correct Answer: C. Myasthenia gravis.**