**Core Concept**
The underlying principle being tested is the diagnosis of postpartum complications, specifically postpartum hemorrhage (PPH) and its causes. **Subinvolution of the uterus** is a key concept here, where the uterus fails to return to its pre-pregnancy size due to incomplete involution.

**Why the Correct Answer is Right**
Given the clinical presentation of bleeding per vaginum on the 22nd postnatal day with a uterus size corresponding to 14-16 weeks and an open internal os, the most likely cause is **subinvolution of the uterus**. This condition often results from retained placental tissue, infection, or poor uterine contraction, leading to incomplete involution and subsequent bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it doesn't directly relate to the clinical scenario provided.
**Option B:** Incorrect as it is not the most likely cause given the specific findings of an open internal os and uterine size.
**Option C:** Incorrect because, although it could be a cause of postpartum bleeding, it doesn't fit as well with the specific details provided in the scenario.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that subinvolution of the uterus is a significant cause of late postpartum hemorrhage, typically presenting after the first two weeks postpartum. The presence of an open internal os and a larger-than-expected uterine size are critical diagnostic clues.

**Correct Answer:** D. Subinvolution of the uterus.